Why do people put politically related bumper stickers on their car?

Vote Obama. Ok, I agree with this, but if a McCain supporter is driving behind you, do you really think your sticker will make him/her have an epiphany and think “By God. I see where I am wrong, Obama it is!” Is the sticker more about putting a bit of who you are and your beliefs “out there”? Why does that matter on the road? Could it lead to subconscious road rage between people supporting different candidates?

Disclaimer – I am not trying to tell you to take off the bumper stickers from your car. I am just curious as to why people put the politically related ones on their car. The reason I ask this Q is because my friend just gave me a Obama bumper sticker and I wasn’t sure I wanted to put it on my car. I am not ashamed of my views, but I do like MY car sticker free.

PS – please DO NOT use this thread to fight about politics or the candidates.
PPS – Moveon is giving away free Obama bumper stickers for anyone who wants it.