I've trimmed fins once due to severe fin rot. Some breeders do it to help the fish heal up easier after spawning because a clean cut heals better than a jagged one if the fins get torn.
Even a well bred fish who carries his fins well can need trimming in older age, it's just a humane thing to do. However I do NOT recommend any person off the street attempt this. You need anesthetic, very SHARP scissors or a scalpel (I use an exacto knife) and you need to know how to treat the fins so they heal up beautifully.