When you need a solid vehicle for your message, why not put your message on a vehicle? In the world of advertising, there is no space that won't be used as a means for promoting a message. The more spots you can find for your ad, your slogan, your cause, or whatever it is you want people to see, the better your product, or company is going to do. Whether it be a car ad, a political message, or any other product or service, a good message needs to be heard in order for it to spread.
The bumper sticker is pretty much the same idea. It is a roving billboard, and it advertises a message to whoever happens to be stuck behind you in traffic. It is a passive message, and allows a person to say something without really saying anything at all. The mini billboard allows a person to show off what is important to them, and what there views may be about anything in life.
During the election season I saw a lot of anti-war bumper stickers, and quite a few Obama '08 bumper stickers driving around my hometown. Clearly, we were pro-democrat as we lived in western New York, and New York is a very democratic state. Also, most people were fed up about the war, and wanted it to end. By putting up a bumper sticker, a person could voice a message in a convenient and cost effective way.
Putting up a bumper sticker can give a person an idea about the culture that they live in, and gives a sense of what people are feeling. If someone has a lot of bumper stickers, it may show that they are very passionate about spreading several messages, or trying to get people to be aware of a topic. You see a lot of bumper stickers that have websites on them, or other messages that allow the reader to get more information on the topic.
A bumper sticker isn't always just to send a message though. Sometimes a bumper sticker just looks cool, and is good for decoration. I put up a couple of Buffalo Bills bumper stickers on my car because I thought they looked cool. I wasn't trying to say anything in particular, just that I like the sticker, and decided to put it on my car.
I once put a bumper sticker on my car to hide some damage that had occurred to my car. It wasn't anything major so I just covered it up with a sticker. The bumper sticker looked cool, and it helped make the exterior look a little less bruised.
When it comes down to it, a bumper sticker works in two ways. It is a good way to decorate your car, or tidy up the exterior, and it is a good way to spread a message. A bumper sticker is another version of an advertisement, and is a good way for the everyday person to spread the word about the things they care about.