Antioxidants are very important in today's cosmetic formulations. Years ago they were put on the ingredient label to make the product look good. I don't know if cosmetics formulators really knew how important antioxidants would be down the road. Antioxidants serve some simple but very useful functions. One thing that scientists agree on is that antioxidants are very important in regulating and sustaining normal cell processes and promotes cell growth.
Antioxidants are not preservatives, but they can control the oxidation of ingredients in a formulation. This oxidation process produces nutrients that microbes thrive on. These by-products of oxidation are not made available to the microorganisms and hence they cannot multiply. Antioxidants work along with preservatives to protect the microbial integrity of the formulation.
Antioxidants also function as free-radical scavengers. Free radicals have been shown to accelerate the visible signs of aging. Antioxidants can help in a formulation and can function as anti-aging ingredients along side the other ingredients that are used to fight the visible signs of aging.