Well the first reason is simple. We live in an acid world!
Most things in and around us every day are acidic. Even we are acidic. Most people’s skin sits at about 5.3 on the pH scale (acidic). The air we breathe out, CO2 is acidic. The most pH balanced thing we deal with every day is water, that sits at about the pH neutral of 7.
Dirt and most pollutants are acidic and therefore since opposites attract, the best cleaners for most applications fall on the alkaline side of the pH scale. This is also the reason two step washing is so popular and works so well, such as in fleet washing. You first apply an acid, followed right behind with an alkaline and 'Voila' the dirt falls off, or is repelled from the surface.
So should we take this approach further into other cleaning applications such as house washing? Maybe yes and maybe no.
Most contractors would prefer to wash anything in as few steps as possible and house washing in no different. Many have even gone so far as to try and make or use single wash products; to clean, kill mold, leave a protector (mold repeller) and or surface enhancer (such as a wax) on the house, like a "Pert Plus" shampoo and conditioner for hair. The problem with this model is that you get less effectiveness and use more of each part when they are put together rather than used separately.
The best method, that would use the least amount of chemical resources, would be to first knock off as much dirt and debris as possible with a stiff stream of hot water, followed with an alkaline soap applied as hot as possible, rinsed, then a bleach mold killer added and rinsed, with a mold protector or surface enhancer like a wax then added at the end.
Now some parts of this cleaning can be put together without effecting results such as adding bleach (12 % Sodium Hypochlorite) to a stable soap to clean and remove mold, algae and mildew. Or adding a wet wax product to rinse water so that a house can effectively washed and protected (waxed) in two separate passes. Now depending on the soil type, soap and water temperature, extra time maybe needed for scrubbing or agitation of the chemicals on the surfaces to get them totally clean before they are rinsed free.
I will go more into the all in one idea for house washing products in my next segment. But for now understand that there is a dance you must do to see if using less of each product, but more in time and labor effort, is worth the cost difference. Rather than combining products that will use more in chemical costs, but less in time and labor, balanced out by the results that can be achieved with each.