Why do paratrooper's parachutes have holes in them?

Answer 1

A parachute works by trapping air underneath, but when the air is trapped the pressure increases and some of the air has to escape. If there were no hole in the center of a round parachute, such as the military 'chutes used in WW2, the excess air would have nowhere to escape except under the edges of the 'chute, which would cause increasingly violent oscillation and quickly the collapse of the 'chute, which would drop the parachutist to the ground at high speed and kill him. The central hole allows a controlled release of the air pressure in the 'chute, preventing oscillation and the collapse of the 'chute.

Answer 2

Parachutes have several uses. They may be used to rescue a pilot from a damaged aircraft, to stop an airplane on a short runway, or to drop soldiers/ supplies/ equipment into areas that cannot be reached by other means. They are used to help avoid that sudden stop at the bottom when falling from 1,000 ft.