Pruning. Proper and regular pruning is fundamental for some trees to keep them from excess growing and to maintain its health. Different trees and situations require different pruning techniques.
Thinning. Makes trees look better and care for while making it healthier. At a home thinning a tree will make your garden just look better, and in commercial sites it shows professional care.
Sudden increased risk. Trees or branches don't need to fall to show an increase risk to do so making its removal urgent.
Tree health. Both pruning and thinning will promote the long term health of a tree as well as helping the tree grow stronger.