Shade trees are often clipped at the nursery and in the landscape later develop numerous competing stems. This makes trees susceptible to mechanical failure (where branches break off large portions of the trunk and can lead to tree death).
In a forest it is more typical to see trees with one dominant leader and small well-spaced branches. For trees in a landscape, structural pruning is a way to reduce and subordinate branches and develop one strong leader trunk.
This video goes into details about how to structure prune your trees for long healthy life in the landscape.