A well-maintained tree or shrub is healthier and looks better - two very good reasons to learn how to prune correctly. Here are the specific reasons.
Maintain safety: Remove low-growing branches if they impede passing vehicles or obscure oncoming traffic from view. You may also need to take out split or broken branches before they have the chance to come crashing down on a person, car, or building. It's wise, too, to prune out low-hanging, whip-like branches (especially those with thorns) that may strike passersby.
Alter or rejuvenate growth: Neglected, overgrown shrubs can sometimes be turned into small multi-trunked trees if you remove their lower limbs; this may be a better approach than digging out the shrub and planting another in its place.
Direct growth: Pruning influences the direction in which a plant grows: Each time you make a cut, you stop growth in one direction and encourage it in another. This principle is important to keep in mind when you train young trees to develop a strong branching structure.
Remove undesirable growth: Prune out unwanted growth periodically. Cut out wayward branches, take out thin growth, remove suckers (stems growing up from the roots) and water sprouts (upright shoots growing from the trunk and branches).
Promote plant health: Trees and shrubs stay healthier if you remove branches that are diseased, dead, pest-ridden, or rubbing together.
Create particular shapes: You can prune a line of closely planted trees or shrubs as a unit to create a hedge. If you're a hobbyist who practices topiary, you can prune trees and shrubs into fanciful shapes.
Produce more flowers or fruits: Flowering plants and some fruit trees are pruned to increase the yield of blossoms and fruit and to improve their quality. You'll need, for example, to remove spent flowers from roses throughout their bloom time. For some fruit trees, you'll make many small, precise cuts each dormant season. Although this sort of pruning sometimes ranks as a tedious chore, remember that your efforts will pay off in lavish bloom and generous crops of fruit at harvest time.