Knee pads with under-or-over compatibility give you the flexibility to wear them however suits your day—snug under your ski pants for a clean look and warmth, or strapped over them when you want quick on/off access or are layering up for cold conditions. This versatility is ideal for all-mountain skiers who deal with changing weather and preferences throughout the season.
The trade-off is that these pads are a middle ground. They won't be as low-profile as pads made exclusively for under-layer wear, so they might feel slightly bulky under slim-fitting pants. They also may lack the heavy-duty straps and quick-release features of dedicated over-layer pads that racers rely on.
For most recreational and all-mountain skiers, though, the convenience of switching it up outweighs the slight compromise. Just make sure they fit comfortably in the configuration you'll use most.