I think it depends on the school. Some of the supplemental application's only purpose, I felt, was to provide the school with some information before they received your OPTOMcas application: GPA, address, e-mail address (and of course 50 bucks) I thought UMSL and SCO were like this. I was under the impression ICO will not offer interviews until the supplemental application is received, I felt their supplemental application was to weed out some applicants since they get so many.
Ohio State offered me an interview before they received my supplemental, and that supplemental was REALLY hard, AND it was the only one with a deadline. It was 4 essays, and more like a medical school supplemental app. I felt like that one, since Ohio State has an open file interview, was to help give the interviewer some more interesting or specific questions to ask at the interview.