Bib chest pockets sit right on your chest, giving you easy access to essentials like your phone, goggles, or snacks without unzipping your jacket. Because they're up high and close to your body, items stay warmer and drier—your phone is way less likely to die from cold compared to sitting in a thigh pocket.
The big advantage is security. When you're hiking, sitting on a lift, or taking a spill, stuff in a chest pocket stays put. You don't have to worry about things falling out when you sit down or move around.
The catch is that bib chest pockets only exist on bib-style pants. If you prefer regular snow pants, you won't have this option. But if you're already considering bibs for the extra coverage and warmth, the chest pocket is one of the best perks of the design—especially if you like keeping your phone accessible for photos on the mountain.