It's designed to allow offensive players freedom of movement and prevent defenders from crowding them or making illegal contact while they are attempting a normal basketball play (shooting, passing, dribbling, or pivoting). 

A defensive player cannot invade the cylinder of an offensive player with the ball and cause illegal contact when the offensive player is attempting a normal basketball play.

If a defensive player invades the cylinder and makes illegal contact, it is a defensive foul.

I'm surprised this wasn't brought up when Dawson Baker was getting assigned a flagrant 2, it was such a strange application of flagrant 2 rules.