A prepcoat is needed when a surface you are intending to paint is not sealed or relatively flat. Also, any surface which is mildewed or stained requires a prepcoat. Bare timber, cement and gyprock are all porous services that require prepcoating for the paint and the surface to bond and for you to achieve good coverage. If a surface has been painted previously you do not need to prepcoat unless the existing paint is in poor condition, you are painting a water-based paint over an oil-based paint or you are using a colour that is in stark contrast to the colour of the existing paint (e.g. white over navy).