Sun is doing two things at once
It is bleaching the surface, which you can see, and it is drawing the oils out of the asphalt, which you cannot. The two happen together, which is why a faded roof is usually also a drying roof.
That is the reason fading is a useful early warning rather than a cosmetic annoyance.
Where the colour went
The granules carry the colour, and the surface around them carries the tone. As granules loosen and the asphalt dries, the roof loses the even colour it had when it was laid.
Slopes facing the sun fade first and furthest. A roof with one bright elevation and one dark one is telling you which side is taking the wear.
What a treatment does, and does not, do
It does not dye the shingles or restore the original colour, and the site says plainly that the roof keeps the colour it has today. What it does is work into the surface so the roof sheds water again and keeps its flexibility.
After treatment a roof usually reads as cleaner rather than recoloured, because the surface is no longer holding dust and moisture.
When fading is worth acting on
Fading on its own is not an emergency. Fading with granules in the gutters, edges beginning to lift, or water sitting on the surface is a roof that has dried out, and that is the point where the score matters.
A roof still holding water out and scoring in a treatable band is exactly the roof the range is built for: it buys years from shingles that still have some.
Faded on one side only
A roof that has faded on the sun-facing slopes and kept its colour on the others is telling you where the wear is concentrated. That is normal, and it is why the score is taken from the roof as a whole rather than from the best-looking elevation.
It also means a photograph taken from the street can flatter or condemn a roof unfairly. The assessment looks at the surface rather than the view.
What is not worth doing about it
Re-colouring a faded roof does not put the oils back, and a coating over the top changes the surface rather than restoring it. Neither changes what the score would say.