Because glutathione protects your skin from strong oxidants!
Glutathione is one of the peptides that matter, and a lot.
Glutathione is present in us and in most living beings for good reasons. Glutathione helps us cope with life at 21% oxygen (what we do have in the atmosphere).
Respiring oxygen allows us get the most energy out of our food but it carries a price: Reactive Oxygen Species (ROS*),
What are ROS*? Reactive oxygen containing molecules that are so desperate for electrons that will take them from anything and everything. There goes your DNA, your proteins, your lipids (vital for cell life) etc.
In top of the ROS* we produce during respiration, there are also those that are present in our environment, especially in city air.
We are used to hear about vitamin C and vitamin E as antioxidants, but we need a lot more than those two to protect you from ROS*. Glutathione is part of the antioxidant system that will do just that.