When you see reference to proteins in hair care and skin care products, the first thing you need to ask is 'What form of protein?'

The main categories of proteins used in personal care products are hydrolyzates and amino acids. You will often find on an ingredient panel reference to some kind of hydrolyzed protein, like hydrolyzed collagen or hydrolyzed soy protein.  These hydrolyzates are film-formers that bind moisture. Good for both hair and skin, the typically large molecules enhance moisture retention, smooth and seal hair surface and add manageability and highlights to the hair. By contrast, amino acids are much smaller pieces of the larger proteins. Due to their size, they are able to offer much better penetration and offer moisture binding to skin and hair. For hair the benefits are increased flexibility and tensile strength.  For skin, the benefits are smooth and nourished skin surface.

Keep in mind that when you see collagen or keratin, these are animal derived.  Most others refer to vegetables in general or specifically.