Lets say youve already gotten into your top choice, or some of your higher ranked schools.
Would you really want to spend all that extra money for attending an interview at a school that you would definitely not choose over one you have already been accepted to.
Or lets say you didnt get the interview yet, but you know there is absolutely no way you would attend because of your other acceptances. By withdrawing, you save the adcom time and effort by taking your file out of the pile they are going through. This also gives another applicant the interview slot that you would have taken.