Within a couple of growing seasons, a nice garden along a fence line can become an over grown mess. In some cases, it may also become a fire hazard. Neighbours may also complain if your trees and shrubs start to overhang onto their property.
By pruning and removing the undergrowth, you allow more light into the area and better air flow. Neighbours are happy, and you reduce your fire risk if your fence line is close to bushland.