I use transpose because I play by ear and have learned to play well in one key. The secret to playing well is to know your chords and make chord changes without having to even think about them.
Nothing wrong with using the transposer but some musicians frown on it as a cheating way to play in other keys.
If you have been formally trained, then you don't have to use the transpose but for most people, it gives us a chance to play in keys we don't have the skills to use.
Music is about having fun-Playing in other keys can have a nice effect by sounding more mellow or if one is singing along it gives the person a chance to sing in their "natural" key.
Try playing songs in other keys than written in, you'll be surprised at the results. Have fun and transpose anytime you feel like it. That's what it's for !
Regards, Lloyd