Trees are pruned for three main reasons.

Safety
Pruning for safety involves pruning branches that could fall and cause injury or property damage, or trimming branches that could interfere with lines of sight on streets or driveways or utility lines.  Safety pruning can largely be avoided by carefully choosing species that will not grow beyond the space available to them and are suitable to the sight. Before you plant your Tree

Health
Pruning for health involves removing diseased or insect-infested wood, thinning the crown to increase airflow and reduce pest problems, or removing crossing or rubbing branches.  Also removing broken or damaged limbs will encourage wound closure.

Aesthetics
Pruning for aesthetics involves enhancing the natural form and character of trees or stimulating flower production.  Pruning for form can be especially important on open-grown trees that do little self pruning.

Before Pruning Remember

• Each cut has the potential to change the growth of the tree.  Always have a purpose in mind before a cut is made. 
• Proper technique is essential.  Poor pruning can cause damage that lasts for the life of the tree.  Learn where and how to make the cuts before picking up the pruning saw. 
• Trees do no heal the way people do.  When a tree is wounded it must grow over and "compartmentalize" the wound.  In effect, the wound is contained within the tree forever. 
• As a rule, small cuts do less damage to the tree than large cuts. This is why proper pruning (training) of young trees is critical. Waiting to prune a tree when it is mature can create the need for large cuts that the tree cannot easily close.  

The International Society of Arborculture Information on Pruning Young Trees