Huge packs have a variety of applications. If you are a climber, you'll need to haul a ton of ropes and gear - climbers frequently hike out into the wilderness, base camp, and go climb a peak. If you are packing extra because you are taking a family or a group of kids (scouts for example). If you are hauling packrafting gear and food for 2-3 weeks, you might need a huge pack. Winter backpacking requires a different gear list altogether - white gas stove setup, lots of clothes, heavier sleeping bag, 4 season tent instead of a lightweight 3 season, etc.

For general 3 season backpacking, you can get away with a whole lot less.