There are a number of reasons why your trees may need pruning.
Pruning for health involves removing diseased or insect-infested wood, thinning the crown to increase airflow and reduce some pest problems, and removing crossing and rubbing branches. Pruning can best be used to encourage trees to develop a strong structure and reduce the likelihood of damage during severe weather. Removing broken or damaged limbs encourages wound closure. 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 very little self-pruning.
Thinning, which opens the foliage of a tree, reduces weight on heavy limbs and can reduce a tree's height; it also helps retain the tree's natural shape.
Cleaning and Deadwooding, which is the removal of dead, dying, diseased, crowded, and weakly attached limbs and watersprouts from a tree.
Raising or Elevating, which is the removal of lower branches in order to provide clearance for buildings, vehicles, pedestrians, etc.