This makes it easier to carry the bag without dragging it on the ground or having to lift it up with one hand while holding your other arm. It also helps keep the pack stable when it’s loaded with things you want to be able to take out quickly and easily.
https://www.fjallbackpacks.com/wp-content/uploads/2022/05/Fjallraven-Backpack-For-the-Whole-Family-1.jpg