A fully waterproof pack with sealed seams uses taped seams and waterproof zippers to keep moisture out completely—even in heavy, wet snow or rain. If you're doing multi-day tours where gear stays in your pack for hours, or skiing in coastal ranges known for wet snow, this level of protection is worth it. It ensures extra layers and electronics stay dry no matter what.
The trade-offs are real, though. Waterproof packs tend to be heavier, pricier, and less breathable, meaning a sweatier back on the uphill. For most day tours in drier conditions, a DWR-coated pack handles normal snow exposure just fine. Even with a waterproof pack, it's smart to use a small dry bag for electronics as extra insurance.