As the wikipedia entry indicates, the warning shot was really meant for ships and aircraft whose radios were not functioning OR where there might be some type of communications barrier. A warning shot makes sense with ships and aircraft.
The effect of a warning shot could be two fold in certain situations on the ground. The target could either go to the ground OR the target might make a last ditch charge at you in which 1) they get to you and take away your weapon or 2) you have to shoot the target before they get to you.
The reason why you would not fire a warning shot on the ground is because you dont know what the target might do next. You think they might just drop to the ground or cease hostility when in reality it might provoke the situation further to where your pistol is either taken from you or the target is fired upon.