Imagine the number of things you do with a leg strap-on. First of all you can fuck more than one person at the same time. You just lay on the bed and one will ride your main strap on, and at least 2 more can ride your leg strap-ons!
Or you can just enjoy a dick sliding into your pussy while you have this strapped onto your leg so you’re also fucking someone at the same time.
Or you can go to the ladies toilet with your “friend”, take this out of your bag, put it on underneath your skirt on the leg your quick, make your “girlfriend” happy, then take it off and store it back in your bag as quickly.
Cool thing overall, use of it is limited only by your sexual fantasies.