Interviewers ask questions to elicit information from you.  That you may already know.  But what kind of information are they looking for?  They are looking for "any kind" of information that can help them decide whether to choose you or not... over someone else.  Don't forget, it's a comparative process.  Even if you do well, if another applicant does better, he or she will get the job. 

To put it simply, interviewers do not restrict themselves to the substance of your words, but also look at how you convey your thoughts and whether you look sincere.  The interview process will also be used to see whether you are a good fit for the organization.  After all candidates are interviewed, the impression that you leave behind will probably be the only thing that remains, along with some notes.

A good interview means leaving a lasting impression on the interviewer(s).