Say you have a bag that's stuffed to the brim. All 16 slots are taken, and you want to move one item around. You have to move something out, shift everything around, and then drag the other item back in.
It would be great if you could drag an item between other items in a full bag and have everything in between shift a step to the left/right, or have items swap spots with each other by dragging one over the other. This way we'll never need to temporarily drop items to reorganise our inventory.