Prune for health.
In some ways, this is the weakest reason for pruning. If we plant the right tree in the right place, tree size should not be an issue. Almost all gardeners, however, have found themselves in the position where a tree grows faster than anticipated into other plants or buildings, and the need to keep that conifer in check arises.
Removing dead, diseased, or damaged branches should be done anytime a problem pops up and can be safely corrected. Removing branches that cross and could potentially rub against each other is important because these injuries can be an entry point for disease. Limbs that appear weak because of poor attachment angles (called a narrow crotch) should also be pruned away. As the tree grows, excess bark accumulates in the crotch. This results in weakness because, over time, more bark and less wood holds the branch to the trunk.
Video: Pruning Conifers for Health
With few exceptions, conifers will not resprout if cut all the way back to the base, unlike many deciduous trees and shrubs. It is possible, nevertheless, to rehabilitate old, unsightly conifers with some drastic and creative pruning that can result in a new, pleasing look.