An integrated whistle is a small emergency whistle built right into the buckle of your chest strap. It's designed as a safety feature so you always have a way to signal for help without digging through your pack.
In a backcountry emergency, a whistle carries much farther than shouting and takes far less energy—critical if you're injured or exhausted. Since it's built into a buckle you're already wearing, it's always accessible and adds zero weight to your setup.
While you may never need it, it's one of those "better to have it and not need it" features. Most backcountry-focused ski packs include one as standard. If you're shopping for a resort-only pack, it's less critical, but still a nice safety bonus that costs you nothing.