Okay, here's my strange-but-true confession. I'll preface it by saying my experience was only with one university, so I don't know for sure if this is universal, but the way it was presented to me suggested it might be:
I worked in the admissions office at my university for a while. Since we had students from all over the country and there was no feasible way for them all to have on-campus interviews, the university had an alumni interview program - you sign up when you apply, then an alumnus in your area contacts you for an in-person interview. You get to ask questions about the school, the alumnus gets to ask questions about you, and they send in a form giving you a score 1-5 and notes about how you were in person.
Confession: my job was to go through and pretty much just throw out everything that was between a 2 and a 4. If the alumnus ranked the student a 5, I noted that in the file before tossing out the form. The only time the admissions officers even saw the interview forms (which sometimes had a page and a half of painstaking notes) was if the alumnus ranked the prospective student as a "1" - the admissions officers wanted to know what it was that made the student such a stand-out bad candidate.
I asked about it, and it turns out alumni donations go up significantly when alumni "get involved." So the interview program has very little to do with helping rank incoming students, and everything to do with the school getting a bigger check. Like I said, I don't know for sure that any other schools do this, but it sounded like it was a pretty common practice.
