They may just want to see if somebody is advocating for you. If a researcher really wants you and the committee would like you to call some attention to this, then they may want this directly on the application. In my experience, though, highly competative programs have gatekeeping admissions committees which outrank the requests of the faculty members. They do this to keep their statistics in check.
I have both contacted researchers and have been contacted by them before an interview. If they don't respond very quickly, I have interpreted that to mean that they are not hungry for students.
I'll comment on asking who to talk to. This is coming from somebody who has interviewed with over fifty professors. A senior faculty member is more likely to grill you and less likely to have time with his or her own students. If you ask to talk to them thinking that their research is interesting, it might be possible that they have assumed a passive role in the research. If you choose a young faculty member to talk to, he or she is usually more likely to dominate the interview by trying to recruit you.
