The thumb crotch is the area where your thumb meets the palm of the glove, and it's one of the most common places where ski gloves blow out. Thumb crotch reinforcement adds an extra panel of durable material right at that stress point, preventing the seam from splitting after repeated pole gripping and general wear.
If you ski frequently, grip your poles tightly, or work as an instructor or patrol, this feature is absolutely worth having. That junction takes a beating every time you plant a pole or grab a handle, and without reinforcement, you'll often see holes form there well before the rest of the glove wears out.
Casual skiers who only get out a few times a season may not notice the difference, but for anyone logging serious days on the mountain, thumb crotch reinforcement can significantly extend the life of your gloves and save you money in the long run.