Why is it important that men leave the toilet seat down?

I think it might just be a polite thing to do, and I honestly do not know how that got started. You know how society has some "rules" about what men do to be polite. Like holding the door open for a woman, opening a car door for a woman, helping a woman into her coat, etc. It isn't that women can't do it themselves, it is just polite for the man to do that. The toilet seat thing might fall into a category of politeness. I do not know how it got there, but I do not know how the other examples did either.