Correctly pruning helps trees and shrubs on many levels:

Pruning helps to maintain strong structure, removes dead and weak parts. It also opens structure to allow more stable growth.

Incorrect Pruning causes many problems in trees:

Flush Cuts (Cutting too close to the main branch/stem) doesn’t allow the tree to recover around the cut correctly leading in time to decay and structural instability and eventually a hazardous tree.

Topping (The removal of too much material from the top of the tree leaving large cuts close to the main system) the tree subsequently puts all of its energy into generating excessive epicormic growth. The large and oversized cuts decay and the growth which eventually becomes branches decays an becomes a hazard.

Tipping or “Heading” is the reducing of length of branches back to unsuitable locations and not allow alternative growth point to take the pklace of the branch being removed. This results in dead/dying branches, excessive growth from cuts and sick and deformed trees.