Most people are not aware of the fact that many cellphones have an FM radio receiver in them. As I understand it, if the cellphone has one of the Broadcom chips in it (don't know if the Infuse does since I haven't seen a teardown report) then this chip provides a combination of recievers, wifi, bluetooth, etc. and it also has a FM radio receiver. Like most devices of this type it uses the headphone as an antenna. I had a Dell Streak before the Infuse and even though they didn't advertise it, the Streak is capable of receiving FM radio and I have used it that way before and it picks up stations pretty well. In fact, the iPhone 3G and 3GS also had this chip but Apple never provided an app to access this feature.
As to why someone would want this feature, there are many times when you don't have and can't get an internet connection to stream a station. Plus streaming fails a lot. When I go to ballgames at my local university, I like to listen to the play by play from the radio because you can get more information than by just watching so there are legitimate uses for an FM radio receiver in a cell phone. It's not a deal breaker for me but it would be a nice addition if the Infuse was capable of it.