Why growth settles where it does
Algae and moss both need moisture that stays put. Shaded slopes, north elevations under trees, and any area where water lingers after rain are where they establish first.
Moss is the more structural of the two: it holds a mat of moisture against the shingle surface, which is the condition that leads to freeze-thaw damage in a cold snap.
What the treatment changes
A treated surface beads water and lets it run. Growth that depends on a damp surface has less to work with, and the site describes the treatment as slowing algae growth rather than eliminating it.
That is a mechanical answer to a moisture problem rather than a chemical one, and it is worth being precise about the difference.
What is not claimed
No biocide claim, no anti-fungal warranty, and no promise that growth will not return. This site does not publish those claims, and a salesperson who adds them is going beyond the product.
The warranty covers the failure the treatment is responsible for: premature peeling, curling or warping caused by the treatment, and water penetration through the treated shingle.
The order of work
Growth that is already established is dealt with as part of getting the roof ready, because the treatment has to reach the shingle rather than the growth on it. The surface is cleaned as part of the visit.
Then the treatment goes on, and the roof is a surface that dries out between storms rather than one that stays damp.
When growth is a symptom
On an old roof, heavy growth often sits on top of a surface that has already dried out and started to hold water. Treating the growth without looking at the shingle underneath treats the symptom.
The assessment scores the roof itself, which is the part that decides whether a treatment is worth applying at all.
The clean-first question
People often ask whether cleaning is enough. On a roof that is otherwise sound it can be, at least for the look of it. On a roof that has dried out, cleaning removes the growth and leaves the reason it appeared.