Active ingredient in lecithin. PC is considered a very good water-binding agent and aids in the penetration of other ingredients into the skin. It absorbs well without feeling greasy or heavy (although other ingredients can perform similarly, including glycerin, ceramides, and hyaluronic acid).
References for this information:
Skin Pharmacology and Applied Skin Physiology, September–October 1999, issue 5, pages 235–246
Journal of Controlled Release, March 1999, issues 1-2, pages 207–214