Let’s take a look at the main purpose of State Legislatures, as discussed by experts.
The state legislatures are the best chance we have save the republic.
From your question, I would infer the viewpoint that with a national legislature the state legislatures are at best redundant and at worst counterproductive. While many in America today look to Washington, D.C. for all of their needs, that is not the legal standing of the federal government.
Federalism vs Federal Supremacy
Federalism – “the distribution of power in an organization (such as a government) between a central authority and the constituent units” — Merriam-Webster Dictionary
The Constitution established a new union of sovereign states and created a central government, what we now call the federal government. The Constitution set the boundaries for that government, establishing a federalist system where powers were specifically distributed between the states and the new United States.
“The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.” – U.S. Constitution, Amendment X
The Constitution establishes a clear boundary between the powers of the states and those of the United States. While that is often ignored today, it is not only the law, it is the supreme law of the land.
“This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made, or which shall be made, under the Authority of the United States, shall be the supreme Law of the Land;” — U.S. Constitution, Article VI, Clause 2
Rather than the federalist system created by Constitution, where power is distributed between the state and federal governments, the traditions of the courts have led to a centralization of power in the federal government, in direct violation of the Constitution that created it. This theory of “federal supremacy”, where the states are mere subdivisions, or worse subjects, of the federal government, has been used to destroy one of the most powerful checks and balances in the American system.
Related: What Is the Purpose of the Constitution?
Checks and Balances
We’ve all be taught in school that our governments have three branches to provide checks against the abuse of power. While this is true, it is only a fraction of the true story. Contrary to the federal supremacist view, the federal government is not supreme over the states because the federal government did not create the states, it is a creation of the states.
First, the states predate the United States.
“That these United Colonies are, and of Right ought to be Free and Independent States;” — Declaration of Independence, July, 1776
The federal government did not exist until the Constitution was drafted, signed (September, 1787), and ratified (June 1788 for the first nine states). And since it took the ratification of the states to form this new government, it’s quite plain that the federal government is a creation of the states, not the other way around as those with a federal supremacist view act.
Furthermore, as the Tenth Amendment states, the federal government legally on has the powers delegated to it by the Constitution. The Tenth Amendment states that powers not delegated to the United States are not delegated to the states, they remain with the states. That means the states had these powers before they were delegated to the United State.
Furthermore, only the states have the power to delegate or remove powers from the United States through the amendment process. Therefore, legally, the states have significantly more power than the United States and its federal government.
Purpose of the State Legislatures
According to the Declaration of Independence, the main purpose of any government is to protect the rights of its citizens.
“We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.–That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed,” — Declaration of Independence
As the creator of the federal government, it is the responsibility of the states to retain control of their creation. Sadly, that duty appears to have been lost to history, but not completely.
The main purpose of the state legislatures is to exercise the legislative power of the people of their state in accordance with a republican form of government. That includes not only legislating domestic issues, but controlling their creation in Washington D.C.
We are seeing evidence of a resurgence of the exercising this legislative power as more and more states pass laws that make it illegal for state agents to assist the federal government in unconstitutional acts and restricting enforcement of federal laws within the state.
In other words, we are seeing states begin to take up their role as a check on the abuses of the federal government, which all starts with a bill in the state’s legislature.
Saving the Republic
“That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, –That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness.” — Declaration of Independence
The American people, through our states, created the federal government and consented to limited and defined powers for it. Those in the federal government have abused their position, assuming powers not consented to by the people, nor ratified by the states.
It is the right, it is the duty, of the people to alter such a lawless government. The legal way to do that is through both state nullification and choosing a better representative to exercise the powers we have delegated to them at both the state and federal level.
If the American people wish to remain a free republic, then we must not only jealously guard the powers of the state, we must hire representatives who will do so as well. Otherwise, we will no longer be the United States of America, we will become the Subject States of America.
We will cease to be unique states, with our own ideas, preferences, and traditions. We will become a monolithic people, ruled from a distant city, with little if any control over the exercise of our sovereign powers. The issues and concerns of Iowa, West Virginia, Montana, and the rest of the central states will be lost to the hew and cry from the big cities.
In short, we will cease to be a federalist group of republics and we will become the democratic dystopia the Framers of our Constitution warned us about.
“We are now forming a Republican form of government. Real liberty is not found in the extremes of democracy but in moderate governments. If we incline too much to democracy we shall soon shoot into a monarchy or some other form of a dictatorship.)” — Alexander Hamilton
“A democracy is nothing more than mob rule, where fifty-one percent of the people may take away the rights of the other forty-nine.” — Thomas Jefferson
“Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote!” — Benjamin Franklin
So what is the main purpose of the state legislatures?
First, to exercise the lawmaking power of the people of the state. Second, to help protect the citizens of their state from the encroachment of an out of control federal government.
The basic function of state legislatures, as for any legislature, is to represent citizens on matters of public policy.
In other words, the basic function of legislatures is to provide a direct responsive link between citizens and the workings of government.
Legislatures were the major systemic innovation that revolutionized government, scrapping the theories and principles of top-down monarchial governance, and replacing those concepts with the theories and principles of ground-up citizen-responsive governance.
One can debate how successfully any specific legislature, including any specific state legislature, has implemented or lived up to citizen-responsive governance. But in theory a well-functioning democratically-elected legislature should represent citizens in four basic ways.
Through elections, citizens get to choose who will advocate for and vote for laws they believe are essential for the public good (policy representation). Citizens get to choose who will lobby and press executive agencies to allocate money and resources to meet the collective needs of their specific district or region (allocative representation).
Citizens get to choose who can bring their personal complaints to the attention of executive agencies (constituency service representation). And lastly, citizens get to choose who will embody their values and demographic identity in the public policy arena (symbolic representation).
All of these representative functions link citizens to government, and that is why legislatures exist in every society that claims any modicum of democratic governance, and why they exist in every state of the United States.