Windsurfing foot straps are one way for you to increase your stability on a board. They are a sort of reverse inheritance from snowboarding. Snowboarding itself is a derivative of surfing, just like windsurfing is. However if the two were said to be brothers, windsurfing is definitely the bigger brother, having been around for a few decades longer.
However, the demands of snowboarding required that it come up with adaptations that were unnecessary in windsurfing. For example, if you wipe out on the ocean, it’s a relatively simple matter to paddle over and retrieve your board. If you’re snowboarding down the mountain, you don’t want to have to hike back up if you fall. That is why snowboarders created the first bindings, which were the idea that spawned windsurfing foot straps.
Windsurfing foot straps are less restrictive than a binding, because they don’t need to provide the same amount of leverage. While snowboarding, the only means for you to interact with your board is by applying pressure with your feet, and so a binding needs to be able to pass all of that pressure from your feet to the board without losing any to slippage. While windsurfing on the other hand, you apply pressure both to the board and to the mast, and many maneuvers result in little or no pressure change to the lower body. Thus the foot straps simply need to keep your fee from sliding around excessively, rather than provide a perfect transfer of energy from your feet to the board.