Why it appears on some slopes and not others
Growth needs moisture and time. A slope that faces away from the sun, sits under trees, or stays damp after rain gives it both, which is why the streaks usually run down one side of a roof rather than all of it.
Streaking that follows the shingle courses is characteristic: the growth runs where water runs.
What it does to the shingle
A dark growth layer holds water on the surface instead of letting it sheet away. A surface that stays damp dries more slowly after every storm, and damp is what freeze-thaw and further growth both work on.
It is a wear factor rather than an instant failure, which is why it is scored alongside granules and curling rather than treated as an emergency.
What a treatment changes
The treatment makes the surface shed water rather than hold it, so growth has less to take hold of, and the site states that it slows algae growth. It is not sold as a biocide that kills what is already there.
Growth that is already established is worth removing, and the assessment is where that gets scoped rather than guessed at.
What we will not claim
No anti-fungal guarantee, no "will never come back" promise and no claim about the chemistry beyond what the range is described as doing. The site is explicit that environmental and biological claims are not made on it.
What is on offer is a treated surface that dries faster, and a written term for the failure the treatment is responsible for.
What to do about it
Have the roof scored rather than cleaned blind. Cleaning a roof that is otherwise sound changes how it looks; treating it changes how it behaves, and only one of those addresses the reason the growth is there.
If the streaks sit over a roof that has already dried out, the score will say so and the treatment will be the one that fits.
When to look at a roof in person
If the streaks are on a slope you can see from the ground, the assessment will see them too, and it will also see the shingle underneath them. That is the part a photograph cannot answer.