Professional plastering is required to ensure the strength, safety, and aesthetic of your internal linings (walls/ceilings).
Some specific examples of when you need a plasterer:
The internal frames are exposed (you can see into the walls).
Cracked or flaking paint
Discolouration on walls/ceiling (mould, water damage, smoke damage)
Warping of the walls/ceiling
Dings or holes in the walls/ceiling
Damaged/cracked cornice
Hole in the wall
Cracked or damaged cornice
Peeling or flaking paint
Renovations/new builds requiring internal linings