Knuckle protection adds a padded or reinforced panel over the knuckles, and it's really designed for specific types of riding. If you're a gate racer brushing slalom poles, a freestyle rider hitting rails and features, or someone who charges through tight trees, knuckle protection can save your hands from painful impacts and abrasion.
For most resort skiers and casual riders, knuckle protection isn't necessary and adds bulk that can make gloves feel less comfortable and slightly harder to move in. It's one of those features that's great when you need it but overkill when you don't.
If you do fall into one of those aggressive riding categories, knuckle-reinforced gloves are absolutely worth considering. Just know that you're trading a bit of dexterity and a lighter feel for impact resistance. Pair it with palm or thumb crotch reinforcement and you'll have a glove that can handle serious abuse on the mountain.