Why do we use a parachute when jumping from a air craft?

Well, I have jumped out of quite a few aircraft without a parachute. They were, at those times, parked firmly on the ground.

If you do happen to be sky diving, the use of a properly sized, rigged and packed 'chute will reduce your terminal velocity to a speed that is not, well, terminal. The 'chute increases your aerodynamic drag. Without the 'chute you will reach a speed of approximately 120 mph. Impacting the ground at that speed will not only leave a lasting impression on you, but, also on the ground.