Many backpacks, such as hiking/hunting/camping/range/assault/sling bags, especially military backpacks, have shoulder straps. But they usually don't come with strap loop to manage the long and loose shoulder straps. The belt loops for pants help with that. The size is just right, allowing you to fold a couple turns to shorten the excess length when strapping.

Belt keepers can also accommodate a variety of widths widths less than 2 inches such as heavy duty radio straps for firefighting, tactical undershirt straps, rifle slings, camera straps for photographers, tactical belts, nylon/canvas/web belt, and even extra straps on dog collars.

Simple solution for controlling long straps/belts. Holds on to loose straps. The excess of the belt cannot swing. The belt loop strap stretches so you can get around obstacles. If you look closely, you can see other uses for it in other scenarios such as outdoors and campgrounds.