Pruning young trees is the most proactive maintenance practice for the future mature tree. Structural pruning while young insures a sound framework to support the large branches of a mature tree.
We remove branches with weak attachments and or branches that will eventually interfere with the sound growth of other branches within the canopy of the tree. It is also done to redirect growth that may in the future obstruct sidewalks, roadways, buildings or other plants.
Young trees are much more tolerant of the removal of live tissue than are mature trees. Pruning and training trees while still young decreases chances of branch failure and the need to remove larger branches in the future.