In my humble opinion, based solely on my experience with our program, the purpose of the question can depend on who is asking. Keep in mind that not all interviewers are very actively involved in the entire recruitment process. While they all submit feedback from their meetings with candidates, not all interviewers attend the final ranking meetings.
Reasons for the question could be to figure out a candidate's potential interest in the program, to gauge the candidate's competitiveness for the program and/or their perceived competitiveness for the other programs where they are interviewing, to get an idea of how competitive the program is and to infer its perceived standing among the year's candidates. Sometimes the impetus for the question may just be simple curiosity.
As far as a program getting an idea of its current standing among its cohorts, we'll also analyze the final NRMP match results to see to which programs our ranked candidates ultimately matched. Looking at those results, for both the highly ranked candidates and those lower on our list, gives us trending insight.
Honestly, you needn't worry about the question. Answer it as vaguely or specifically as you'd like. I haven't yet seen the information impact our final ROL.