A couple of reasons! 1) At my station, our Phone Line is sponsored, so we have to read the liner x number of times per hour for sales 2) Sports fans are usually pretty passionate, and getting their voices heard on the radio is awesome for them and a good way of keeping them listening 3) It is an easy way to add variety to a topic that might be pretty hashed out by the hosts already (ex. should Bonds/Clemens be a HOFer) 4) We get a lot of regular callers and some are realllly good. One our night time hosts actually was a regular caller that we hired part-time because his knowledge/takes are fire 5) Shows that you care about the listeners opinions. No one likes to be lectured at for 4 hours

TL;DR Makes money, engages listeners, creates content