Almost all new gymnasts that do their forward roll will just tuck their head in and fall forward. This is fine for learning the idea behind the forward roll, but what we want is the gymnast to push off with their feet to get power into the roll.
The idea behind this drill is to remove the option of just falling forward. Using a wedge or block and placing the gymnast in front will remove the possibility of the gymnast just falling forward. Now the gymnast needs to push hard down on the floor with both feet to make a little jump to initiate the roll.