A gallon of good exterior paint will hold up for ten years on a surface it can actually bond to. On a surface with chalk, pollen, mildew, and road dust on it, the same gallon is a two-year paint job, because the paint is stuck to the dirt and the dirt is stuck to nothing.
That is why the house gets washed first, every time. Around here it is not optional: our springs coat everything in pollen and our summers grow mildew on any siding that faces north or sits under a tree line. A wash removes the film, kills the growth, and lets you actually see the condition of the substrate.
Once it is clean and dry, the real prep starts. Loose and peeling paint gets scraped back to a sound edge. Bare wood gets spot-primed so it does not drink the finish coat. Failed caulk at trim, corners, and window joints comes out and gets replaced, because those joints are where water gets in and starts the next failure.
Rot is the thing homeowners are most surprised by, usually at the bottom of trim boards, at fascia ends, and under window sills. It has to be cut out and replaced before paint, not painted over. Paint is not a structural product.
It is also the reason we discount the wash when it is booked with the painting: you are paying for it either way as part of a proper job, so it may as well be on the estimate as a line you can see. Right now that wash is 15% off with any exterior paint job.