Pre-curved knee pads are shaped to match the natural bend of your knee right out of the box, rather than being flat. This means the padding already follows the curve of your leg, so it sits more naturally against your body when you're in a skiing stance.
The biggest advantage is comfort. Flat pads tend to bunch up behind your knee when you bend, which can feel awkward and even restrict movement after a few hours on the slopes. Pre-curved designs eliminate most of that bunching, so the pad moves with you instead of fighting against you.
For most skiers—especially all-mountain and performance skiers who spend the day bending, carving, and tucking—pre-curved is the sweet spot. It's more comfortable than flat padding and more affordable than hinged designs, which are really built for racers who need maximum mobility with heavy protection. If you're skiing aggressively or just hate the feel of gear that shifts around, pre-curved is worth the upgrade.