This was one of our topics in the College Roadtrip JTerm class. (Photo above from our visit to UD.) For many colleges, the amount of interest a student shows through campus visitations, admissions contacts, interviews, and college fair visits are tracked to measure interest. Those showing greater interest will receive preference in admissions decisions. Here is an article addressing the topic.
Updated with new, more informative article: http://www.mefa.org/importance-showing-interest-admissions-process/