Each admissions office treats interviews differently. If a college offers interviews, they are telling you they value the insights they can gain. In an admissions office full of folders of qualified students, an interview can be a very valuable tool to demonstrate your potential in a three-dimensional way. Generally, the more selective the college, the more applicants they will see who are qualified. The more qualified applicants, the greater the potential benefit of an interview.
Interview only if you're prepared. The purpose of nearly all interviews is to evaluate you. Applicants who take this part of the college's evaluation seriously can improve their chances.
There's no substitute for rehearsing with someone who has conducted interviews. We at CollegeLab have interviewed numerous high school graduates for college admissions. This is an area where we can help applicants clarify how they present their strengths and aid their chances of admission.