The reason why we tip prune; or as it is also known, dead-head plants is to prolong and maximise their flowering.
The reason why tip pruning prolongs flowering is due to the reason why plants flower in the first place - plants flower and then set seed in order to reproduce. Once they set seed, that stage of their life-cycle is complete and there is then no further reason to produce more flowers - their energy is being put into seed production, not flower production. For a perennial plant, this means until the next season; for an annual plant, this means ever again - their job is now done and once the seeds have been dispersed, it will be up to the next generation to continue the species.
So, tip-pruning prevents energy from being directed in to seed setting, and therefore encourages the development of more flowers - in a way, the plant is being tricked into producing more flowers because it believes it needs to do this to produce seeds for reproduction.
This process varies from one species to another, but generally, you must remove more than just the flower petals. It is important to remove all the remnants of the flower - most importantly this includes the mature seed pod or fruit. To be certain, identify the stalk of the flower and cut or pinch it off at its base.