Humectants (hygroscopic) work by pulling water from the dermis to the epidermis. This process increases the level of moisture in the stratum corneum. If the humidity is over 70% humectants can even draw water vapor form the air to help moisturize the skin.
They are added to products to mimic the role of natural hydrophilic humectants in the stratum corneum. The unique structure of the stratum corneum of the skin contributes to its function as a barrier to water loss and the external harsh environment. The injury to this barrier by the environment and common irritants with the resulting loss of water from the skin is the main reason for the development of dry skin or irritant dermatitis. Moisturizers containing humectants can help to increase the hydration of the skin and possibly repair/restore the barrier through use of chemicals that are similar to the skin’s natural moisturizing factors or occlusion of the skin to prevent water loss.
There is a large variety of very different compounds providing moisturizing effects including proteins, acids, polysaccharides, and various small molecules (e.g. glycerine, sorbitol, urea, aloe vera etc.).
Synthetic humectants are able to lock in moisture; however, they don’t provide any nutrients or benefits to the skin. In some cases, they can end up drying the skin over long-term use. It’s always a question of percentage and use.
They serve as a dual purpose, drawing moisture to the surface of the skin while enhancing the skins own hydrating ability. (It can be confusing as glycerin and hyaluronic acid may be derived from animal sources, although most are vegan synthetic versions).