Think of it as going to the doctor for a check-up. You aren't sick, but you want to make sure that everything is functioning the way it should. It's the same for guitars. Every now and then they need the "set-up" checked. This is all the funstuff like height of action to suit your style of play, intonation checks; 12th fret stuff, and other things like fret wear, etc... I think you have the picture now. I personally try to get my guitars into the "guitar doctor" at least once a year. It's probably more for the guys who play a lot!
Bill B.