There are a couple of reasons why it is very important for a person to utilize his or her right to vote. First of all, on an idealistic note, it's a right and a privilege to choose one's leaders in this country, and it is one that many others around the world do not enjoy. Even in some countries where elections are held, often those elections are simply window dressing, or the results may have been tampered with, or the votes may have been influenced through intimidation and fear tactics. In the United States, all citizens over age 18 have the right to vote unless said citizen(s) is a convicted felon.
On the practical side, busy Americans can sometimes be heard proclaiming that "My vote is just one vote. It doesn't really matter." However, those who don't vote have just as much an impact on an election as those who do, because each person who stays home is withholding a vote for one or the other person. An election's outcome can easily be determined just as much by who didn't get votes as who did. This is one reason some have argued that media coverage and projections are not a good idea; some people, hearing a projected winner, might choose at that time not to vote. If enough people make their decisions based on projections, the projection becomes a self-fulfilling one when it might not necessarily have been.