Purpose of humanities PhD interview for less prestigious program? everyone—

A quick and vague question. I've applied to 9 English PhD programs this cycle at institutions that vary greatly in prestige. Only one of them, the least prestigious program, requires a brief video interview for "finalists" (about 15 minutes), in which the candidate discusses his/her interest in the school, academic interests, and career plans with a member of the department.

I was just selected for this yesterday, and the interview is tomorrow! I wanted to know if anyone has any insight about the true purpose of such an interview, which is rare in the humanities, and seems redundant considering that the proposed topics of discussion are all covered at length in my application. It's especially bizarre considering it's not an especially prestigious program/their funding isn't great/etc., so such a step seems less necessary than it would at very competitive programs.

It occurrs to me that they may want to ensure that finalists are genuinely interested in their program before potentially wasting offers on candidates who regard the school as a backup? Of course I'm taking the interview prep seriously, but any extra insight that would help me be more strategic would be much appreciated.
One possible purpose of the interview, from their point of view, is that having an actual conversation may help you become more interested in joining their program.  So it may be partly for recruitment as well as screening.  

This would also explain why a less prestigious program would want to do it: they have a harder time recruiting.