Nearly every exterior in the Midlands is carrying three things a new coat cannot stick to: chalk from the old paint breaking down under UV, a film of mildew and algae on the shaded elevations, and a season of pollen and road dust over the top of both. Paint applied over that layer bonds to the layer, not to the house. When the layer lets go, the paint goes with it.
So the wash is not a courtesy step before the real work. It is the first structural part of the job, and any exterior estimate that does not include it is quietly assuming someone else did it.
There is a right and a wrong way to do it. High pressure on painted wood siding forces water up behind the boards, blows out soft caulk, and can etch the surface into something no coating will lie flat over. Soft washing is the correct method: a cleaning solution that actually kills the organic growth, real dwell time so it works, and then a low-pressure rinse. Hard surfaces are different. Concrete driveways, sidewalks, and brick can take genuine pressure, and they usually need it.
The step people skip is drying. Wood siding needs to give up the moisture it absorbed during the wash before anything gets primed, which in humid weather can mean waiting a couple of days rather than a couple of hours. Painting over damp wood traps that water under the film, and it comes back out as blisters the first hot week.
Washing is also worth doing on its own. A soft wash on the siding and a pressure wash on the drive and walkways will make most houses look several years younger, and if you are not ready to repaint yet, it buys the existing finish time by removing what is eating it. We bundle it at a discount when it is the first stage of an exterior paint job.