This is information that the committee uses to see which schools applicants consider "the competition" and can come in to play, for example, if you are a highly desirable candidate to whom they might want to consider offering you financial incentive to choose thier school. Some applicants get a little freaked out about passing on this information, but as a former committe member at a top five school, I can honestly say this information was never used against the applicants and as far as I recall, would only have entered into the decision making process in cases that it would have benefitted the applicants. Hope this helps.

would it effect my application if I don't put in anything ?

nope, shouldn't affect your application to withold that information, some, however think it an advantage to let them know other schools they are considering, especially if those other schools are well respected. Not a problem at all to leave it out, though, so don't sweat it.