However, a college that offers full ride scholarships may want the FAFSA filed so that it can deduct the amount of Pell grant from the scholarship awarded to a student who is eligible to receive a Pell grant. I.e. the student gets a full ride, but the government pays for a small part of it.
Of course, it does not matter if the student and family know they are not going to get a Pell grant, but the college wants to be sure of that.