It's a fair question! Knee pads with significant mobility restriction feel stiff and resist deep bending, which sounds like a downside — and for most skiers, it is. But for competitive racers doing gate training, that bulk and stiffness is the trade-off for maximum impact protection. Slalom and GS racers regularly smash their knees against hard plastic gates, so they need heavy-duty padding that can take that abuse lap after lap.
The catch is that these pads will change how you ski. Deep knee flexion feels noticeably harder, and you may need to adjust your technique to work around the stiffness. That's fine if you're training gates and need that armor, but frustrating if you're just freeskiing or cruising groomers.
Most skiers — even aggressive all-mountain and freeride skiers — are better off with pads rated as slight restriction or unrestricted. You'll get solid protection without feeling like you're wearing a brace. Save the significantly restrictive pads for the race course, where the protection truly outweighs the mobility cost.