Mildew and algae build up on siding, decks, and walkways, especially on shaded sides of the house. Power washing once a year keeps them under control.
Spring is a good time, after winter grime, and early fall works well before the cold sets in. It is also the right first step before painting or staining, since paint will not hold on a dirty surface.
Pressure matters. Wood, older siding, and soft surfaces need lower pressure and the right nozzle, or they can be damaged. If you are unsure, have a professional do it.