Question

Why do paratroopers roll on landing?

Answer 1

When you hit the ground, the ground exerts a force up on you to stop you. The force which it exerts depends on how long the collision lasts.You have no control over your mass or the speed you hit the ground, but you can increase the time and reduce the rate of change in momentum and rolling is one such trick.Hence, we would get less hurt.
OR
Simply put, it is to minimise the forces on your ankles/legs to prevent injury.