The Idea of the screen in the bottom (and holes in the side) is to air prune the roots. Works a lot like topping a plant to make it grow more secondary stems, the tap root hits air and stops growing down and starts to send out secondary roots that spread out in the container rather than the roots hitting the sides/bottom and circling the pot, getting root-bound. It is basically a way to fill the entire volume of the pot with a good even spread of roots, giving you extra large plants for the pot size. It is used commercially for a lot of things like nursery trees, so they can fit more product into one area...