A full scuff guard is an extended panel of tough, extra-durable fabric that runs from the cuff up the inner leg of your snow pants. It's built to take a beating from sharp ski or board edges, boot buckles, and abrasion where your pants see the most wear and tear.
This feature really shines for aggressive riders, racers, and ski patrol who are constantly pushing their gear hard. The inner leg area gets shredded faster than anywhere else from edge contact during carves and crashes, and a full scuff guard can add seasons to the life of your pants.
The tradeoff is a small weight increase and sometimes a slightly stiffer feel around the lower leg. For casual resort riders or those who stick to groomed runs, standard cuff reinforcement is usually plenty. But if you ride hard, ride often, or are just tough on your gear, the full scuff guard is absolutely worth it for the durability payoff.