Reference tactical belts in general, there’s typically an inner belt that fits inside belt loops, then a bigger outer belt that Velcroes onto the inner belt. The outer belt is what you attach gear to, and keepers are required partially because the belt loops of the gear obstruct the Velcro, weakening the connection. As to why you don’t just throw all that stuff on the inner belt, I think it’s a question of ergonomics and space required. Not 100% on that one