During the primary elections in the Spring, a friend of mine was walking around the neighborhood asking people to put a yard sign on their lawn in support of his favored candidate. By the end of the week, most of the neighborhood was dotted with yard signs. He was doing this 7 or 8 months before the general election. Why?
Yard signs are good advertising, of course. If a candidates name is all over the place, then it can increase people's sense of the popularity of that candidate. It is a standard concept in advertising that reach and frequency increase people's awareness of a candidate (or brand of products for that matter).
But, putting up a yard sign can have an affect on the person who puts out the yard sign as well. In particular, getting someone to put up a yard sign may solidify their commitment to a particular candidate. This process is called escalation of commitment.