There’s a lot of mythology about State of the Union speeches, but we also know a lot about what they do — and what they really can’t do. While we prepare for this year’s iteration, here’s a quick rundown:

In terms of its effects on voters, State of the Union speeches do not, and almost certainly cannot, change people’s perceptions of the president. Most people who watch already have strong opinions about the president, and see the speech through that lens; the result is that it almost always just confirms what they believe coming in. Both political scientists and smart pollsters agree on this one.

Similarly, State of the Union speeches do not, and almost certainly in most circumstances cannot, change people’s minds about public policy. They can, however, raise the profile of issues in the press for a limited time. And they are probably also part of the process of “teaching” partisan voters what to be for and what to be against.

In short, the State of the Union Speech isn’t particularly important as far as its effect on voters is concerned. But there’s more to politics than public opinion and elections, and so the State of the Union actually is important in other ways:

* Along with the budget process that produces the administration’s budget early in the year, the State of the Union is a deadline that forces presidents and their administrations to make decisions about legislative and other goals for the year. (This in turn produces competition to get SOTU mentions).

* The fact that the SOTU will happen appears to push presidents towards finding programs big enough to fill in headlines. That can even matter with a president such as Barack Obama who already has a long list of ambitious programs, but is pushed by the speech to find new ones in order to avoid criticism of “rehashed” or “warmed over” ideas. It may matter even more with presidents who have smaller ongoing agendas in the first place.

* And Matt Glassman makes the excellent point that the pomp and drama of the State of the Union makes it an important democratic ceremony, regardless of any particular policy or electoral effects.

So, yes, the State of the Union really does matter, in both substantive and symbolic ways, just not in the way that people sometimes think it does.