In its old strict sense the term was only applied to the bombardment of defenceless or undefended objects, houses, public buildings. The purpose is to destroy the courage of your enemies. Civilians in a besieged town would put pressure on their military leaders to surrender before the place was destroyed. The practice of using artillery to do this was especially common up until World War I. Since then long range artillery bombardment has been joined by aerial bombardment delivered by aircraft and missiles.