Why do graduate school application forms ask applicants to name the other schools to which he/she is applying?
One thing that some programs look at is whether you are likely to come if they accept you. Say you have a 4.0 gpa, perfect GREs, great letters of recommendation, and publications in respected journals from your undergrad research, and you apply to a bunch of the top schools and some mediocre schools. If there isn't something in your personal statement explaining why you are applying to the mediocre school, and that school sees all the other great schools you applied to, that school may not accept you because they assume even if they did, you would surely go elsewhere. Why would a university do this? Well its a waste of money to fly you out, but more importantly when someone gets put on the wait-list awaiting your decline of the offer, that person may accept other offers from schools that did not wait-list them. I have been told first hand by members of admission committees that the "where else did you apply to" question is used at least partially for this purpose. 

Additionally, at the university administration level, they like to keep track of which universities they compete with for students. I have seen Universities present statistics on how many students reject their offers, and which universities they end up in. So the question is also probably there for statistics tracking purposes unrelated to the admission process. This is a potential reason why even the top schools will ask this question.