A fixed padded hip belt is permanently attached to the pack and can't be removed. Its biggest advantage is excellent load transfer—because the belt is integrated into the pack's structure, it does a superior job of moving weight from your shoulders down to your hips. This makes a real difference on long tours when you're carrying avy gear, water, layers, and extras.
You'll typically find fixed padded belts on dedicated touring packs designed for full days in the backcountry. If you're logging serious vert with 25L or more of gear, that solid load transfer keeps fatigue at bay better than other options.
The tradeoff is chairlift comfort. A fixed belt can dig into your sides when you're sitting on a lift, and you can't remove it to solve the problem. If you split your time between touring and resort days, a removable padded belt gives you similar load transfer with more versatility. But if your pack lives in the backcountry, fixed is hard to beat.