A detachable powder skirt gives you the best of both worlds. You can zip it in when you're riding deep powder or expecting falls, then remove it for warm spring days or groomer laps when you don't want the extra bulk around your waist.
The main advantage is versatility. If you ride in varied conditions throughout the season—chasing storms one weekend and cruising bluebird groomers the next—a detachable skirt lets one jacket handle it all comfortably. Without it, a fixed skirt can feel constricting and trap heat when you don't need the snow seal.
The one trade-off is that the zipper adds a potential leak point in heavy moisture, though quality jackets use waterproof zips to minimize this. If you mostly ride deep powder, a fixed skirt is simpler and always ready. But if your conditions vary, detachable is hard to beat.