I think they're trying to get a gauge on where the applicant wants to practice. Several applicants go to a program because its the place they want to settle down.
Some of it is because they might be able to see other patterns. E.g. if an applicant asked for programs in big cities--it may point out that person wants to live in a big city. Some patterns I noticed--will the program get them into a fellowship-does it have an attached fellowship?, good psychotherapy?, research?
Some also don't like to see an applicant they want apply to too many other programs. It makes it harder for the program to lock onto a candidate they want. When I did my fellowship interviews, several of the interviewers asked the question, and with each additional place I gave them, I could see their smile noticeably lessen.