There are various reasons why drivers may apply bumper stickers to their cars. The most obvious one perhaps, is that they have something to express which they want others to know about. A less obvious reason may be that they are seeking to make their car into their own sacred space which reflects their personality, or that they actually feel a bond with their car more when they personalize it.
It isn't an usual occurrence to find drivers using their cars for advertising purposes. As they often drive long distances in their cars, and park in various places, they can expect quite a few people to see what they are promoting, whether it be their own business or the sale of the actual car in question.
Getting a Message Across
Another reason why drivers may display a bumper sticker may be to get a message across to others about a subject that they feel strongly about. Bumper sticks are available which tell observers everything from whether you believe in God to whether you want to promote world peace or sugar puff breakfast cereals.
It may be that drivers sometimes want to amuse others, or share their own sense of humor. Stickers which have funny sayings or slogans, or possibly are in cartoon form with jokes attached can brighten the day of other motorists when stuck in a traffic jam.
If you have children then you may well opt to have a bumper sticker which lets other drivers know that you have a child on board. The idea is that it will make them think, and hopefully slow down. The sticker may also make it more obvious to rescuers, should you have a crash, that they need to look for a child in the wreckage.
Moralistic stickers tell observers to stay on their guard. They remind them to be careful and considerate and to keep their speed down as they are too close to your bumper if they can read the sticker on it.
Telling others via a bumper sticker who to vote for probably will not influence them greatly. However, it helps to unite the owner of the bumper sticker with like minded folk and to make their views public.
Sweet and cute stickers are there to give the message to observers that the owner of the car is a big softy. Using sticky sweet bumper stickers is a way of saying, 'be nice to me, because I am a nice person.'
Stickers which state particular information with regards to a drivers personality are there as a form of social networking, so that others know a little about them in a similar way to how a My space or Face book biography section works on the Internet.