What's the deal with bumper stickers?

As I was driving home from work today I ended up behind someone who had a pair of Obama/Biden bumper stickers on his car.  That got me thinking.  What is the purpose of political bumper stickers?Are they trying to convince others to vote for that candidate?  I mean, do people really see political bumper stickers on the back of a car and think to themselves, "You know, that guy supports Obama.  I think I will too!"?  Or is it purely to show your support for a candidate?  Because if it is, I have to think there are better ways to do it.  A bumper sticker is a pretty permanent show of support.  Four years later, you just look silly.  But to each his own I guess.