Looked at in the most generous light they simply serve no purpose. Looked at less generously they just exist to piss people off.
'Wow. You're bumper sticker has totally changed my mind about President Obama/Abortion/The United States' role in the UN!' -No one, ever.
So it can be taken as read that yesterday morning, as I was driving to work, my world was comprehensively not rocked by the astounding information that the driver of the car in front of me apparently 'Loves (their) Chow-Chow'. Or more accurately, hearts their chow-chow.
It did, however, get me thinking about what it is about bumper stickers that irritates me. I think it boils down to the fact that they can essentially be broken down into three broad categories.
1. Information that has no relevance to my life.
See, for example, the above example re: love of chow-chow. I'm sure you love your dog, dogs are awesome. I have no need to know that about you while we drive down the interstate. Try loving your turn signal for a change, how about that.
Another pervasive and egregious example of this type are those incredibly stupid little stick people that one can put on the back of ones SUV in an effort to assist any pedophiles that might want some handy information about your children.
2. Something you think is funny
Maybe it is, maybe it isn't, I couldn't say. Because it's a paragraph long and printed in 12 point font on a vehicle 20 feet ahead of me. By the time I've spent 10 miles trying to make out what it says I'll probably have rear ended you, which will not make either of us laugh.
And even if your incredibly funny bumper sticker is actually brief and legible enough for me to read, it is not going to be in any way funny. This is because the bumper sticker environment is actually toxic to comedy. Anything genuinely funny immediately withers and dies when it's brought within 5 feet of a bumper. This is why there are no drive in standup comedy venues. In any case, most stand up comedians are perfectly capable of not being funny all on their own, thank you.
3. Something designed to piss off half the people who read it.
Politics. Religion. The relative merits of sporting franchises. These are all items that people feel compelled to put on their bumper for some reason, despite the fact that half the people reading it will already agree with your point and so you've just wasted their time and the other half are just fantasizing about smacking you off the side of the road like you were one of the motorcycles in Spyhunter*
*Best. Videogame. Ever. A driving game, you were a spy in a tricked out spy-car who had to avoid/destroy villains cars while not injuring innocent bystanders. Pulling alongside a motorcyclist and smacking them off the freeway with a quick jerk of the steering wheel was just about the most satisfying thing in the whole world, although it did cost you points.