The D4+2.0 suspension can only be adjusted in the rear, though it is a relatively simple process. Here is a video highlighting the general process of adjusting the ride height of a coil shock for the D6+. It is the same method used for adjusting the rear shock on a D4+ 2.0:
The main reason why you would lower ride height is to increase overall ride stability at higher speeds. This is done by lowering the center of gravity on the scooter. Reducing the height of the shock will reduce the overall travel and clearance of the scooter, but this is just the tradeoff of a lower suspension. It is always recommended to adjust the suspension so that the front shock is slightly higher than the rear, even though the front suspension length is not adjustable on the D4+2.0.
A common misconception about adjusting ride height is that it will affect the compression rate of the shock. This is untrue, since the coil itself is the only factor that affects compression rate is the spring constant, which varies by the actual coil, not the length. The only thing that will change by reducing ride height is the reduced clearance and lowered travel.