Widely used in the cosmetic and food industry stearic acid acts as a binding and thickening agent between oil and water. It is not a stand alone emulsifying agent, but rather an ingredient used in conjunction with another emulsion agent, like SCG or emulsifying wax, to create one heck of a quality lotion. Your formulation doesn't NEED to have stearic acid in it, but it will help provide body, texture and impart a visual pearl like presentation to your finished product. I like to add this at about a 3-4% ratio ... I've noticed that going beyond that, the stearic acid generally leaves behind a waxy like feel on the surface of the skin that you don't get when added at a lower ratio.
In cold process or melt & pour soap recipes, you can add a small amount of stearic acid. It will help provide your bar with a harder finished texture. For melt & pours, keep at a very low ratio - generally 1/2 tsp. per pound of soap. The more you add beyond that - the lathering ability of the soap will begin to decrease. Also used in candle making for the same purposes: to harden the candle for a slower burn / longer life.
Our stearic acid is derived from palm oil - it is not a synthetically produced product.