For admission to many colleges, an interview is optional. This does not mean that the school does not value the interview. For many schools, it simply isn't practical to conduct an interview with every applicant. Many admissions offices don't have the staff to interview thousands of students, and on the student side, traveling to a college takes lots of time and money. While some colleges rely on alumni to conduct off-site interviews, this practice is also not feasible for all schools. The result is the widespread practice of optional interviews.
There are, however, several reasons to do an optional interview. At many schools, demonstrated interest is a factor in the admissions decision. An easy way to express interest is to show up in person for an interview. Also, the interview allows the admissions officers to get a much better sense of your passions and personality. A list of extracurricular activities and SAT scores doesn't really reveal who you are. An interview allows the college to attach a real person to the application.
Also, it's important to remember that an interview is a two-way exchange. Nearly all interviewers provide an opportunity for you to ask questions, and you are likely to leave the interview with a much better sense of the school than you had before. To learn more, read the article on reasons to do (and not do) an optional interview.