Why homeowners use one
A written decision can be useful where a future buyer, lender or conveyancer may otherwise have to decide whether the extension was permitted development.
Drawings still need to be accurate
The certificate relates to the proposal shown. Changing height, footprint or roof form during construction can undermine the value of the decision.
A drawing can make a detail look settled before anyone has checked the existing building. Measurements, drains and structure still need confirming on site.
It is not Building Regulations approval
Planning lawfulness does not confirm structural design, insulation, drainage, fire safety or electrical compliance. Those sit under separate controls.