This design is made with the intent of improving the safety of pillion riders on two wheelers. In the event of accident (front on collision) or sudden braking at high speed, the pillion rider is the first one to get thrown off due to sufficient/improper holding on the bike's supports and is in the highest danger of fatally wounded. This design is just like the seat belt in four wheelers except it holds the leg firmly to the bike's leg rest and considerably reduces the chances of the pillion rider getting thrown off from the seat.
It also prevents the rider from getting injured in the leg during a fall with the protective paddings.