Inquiry letters have a number of advantages. Obviously, you are able to get specific information that will help you make the best decision. You may be able to contact an athletic department or find out about specific scholarships offered by the college/university. The inquiry letter also establishes a paper trail and helps you make consistent contact with the admissions office.
This paper trail can have a very positive effect on your application. Admission committees want to admit people that seem interested in attending, so they frequently keep track of how often you contact a school as a way of measuring your interest. Of course, you should not go overboard with inquiry letters and send them all over the campus.
If you plan to send the inquiry letter via email, make sure to include a very clear subject line. Also, be certain to compose your email as carefully as if you were writing a formal letter.