Acids in cosmetics get sorted by strength, which is a rough guide at best. AHAs such as glycolic acid are small molecules that move into the skin quickly. BHAs are oil-soluble, so they go where the oil is. Polyhydroxy acids are the third family, and what defines them is size.
A PHA molecule is larger than an AHA molecule, so it moves through the skin more slowly and does more of its work near the surface, and that single physical difference is why PHAs tend to be the ones suggested for skin that reacts to everything else.
Gluconolactone is the PHA we use, and it's a humectant as well, which means it holds water rather than only working on the feel of the surface.
It has a third property that has nothing to do with skin at all. Gluconolactone binds metal ions, which makes it a chelating agent, and in a shampoo that means it helps deal with the minerals hard water leaves on the hair.
That's why it sits in our shampoo rather than only in a face formula. One ingredient covering gentle surface work and mineral binding at the same time is unusual enough to be worth naming.
Gentle is not the same as inactive, and it isn't a reason to use more. PHAs are still acids, and skin that's already irritated usually does better with a pause than with a milder acid applied more often.