Wow - certainly brought out the comments from the pressure washing companies here. All I can say is it sounds like they protestith too loud - a bit sensitive about the quality of their work maybe.
There are certainly some good cleaning companies out there - many of the old-time professional ones that do commercial building especially. However, from my experience I have a generally low regard for the "profession", because I would say probably about 80-90% (whether members of a pressure washing association or not) do a marginal to substandard job. The ultimate test is the white glove test - take a white wet rag or paper towel and wipe the surface when they are just "done" - in a vast majority of cases it will come away seriously dirty, sometimes not even at all white any more - black or gray, showing the "cleaning" was more a wetting than a true "cleaning".
For proper cleaning of a surface, especially before painting, yes they push just pressure washing as the cheapest. But that is sort of like putting some soapy water in the sink or on a dirty plate and dipping the dishes in it and hitting it with the spray nozzle, without any scrubbing - does not do a very good job. To get a truly clean surface, it should initially be wetted (so the detergent surfactant will spread evenly over the entire surface, then detergent solution (and algae or mold remover and/or algaecide/fungicide if needed) sprayed on it, let sit the right amount if time for the product to let it act on the dirt or growth, then HAND SCRUBBED to break up and actually remove the dirt from the surface and to scrub out the deep-seated dirt and staining. This step is "old school" as a couple of the comments said - but just because the "current practices" ignore it does not make them right. Then commonly a second wash with detergent solution and a second pass with scrub brushes (always "with" the lay of the product, never "across grain"), then jet or pressure washing to remove the remaining dirt and chemicals.
Yes it will cost twice as much as a typical pressure washing job, which might be OK for a general house cleaning to make it look cleanear, but at least for a repaint I strongly recommend a hands-on scrubbing job.
That is the prep method that gives you 20-30 year paint adherance for an older or dirty house.
And yes I know about "soft cleaning" - just another way of saying use only as much pressure as needed for the job without risking cutting into the building products.
One the comment I made of the photo showing the initial wash pass being done vertically on the siding - I stand by my comment - any cleaner that used passes crosswise to the lay of the product would be booted off my job for incompetence, because it makes for a visible streaking that will commonly never go away and can even shadow through single-coat paintjobs.
The screaming about using ladders or scaffolding or manlift from several contributors shows the general mindset of the pressure washing companies - they have no idea of the damage they are doing by pressure washing from ground level on lap type sidings, and have probably never seen the water infiltration into the wall. I have frequently seen pressure washing companies blithly spraying from ground level and driving the water into the eave openings and attic (sometimes to the point of staining of ceiling drywall) when washing the top of the walls or the underside of the roof overhang, and also it is common to see them wash a wall and move on, then see water dripping out from under the siding along the top of the foundation and above windows and doors for a couple of minutes, revealing the water that penetrated up under the lap siding while they were washing. Lap sidings, with VERY few exceptions, have little or no waterproofing against water coming up under them, so it just penetrates right to the water barrier - if you are lucky enough to have one, or into the insulation and wall if not or if the barrier is old and deteriorated. Plain and simple - with lap type products, the water should hit the surface downward, not upward, to prevent water infiltration.