Sodium Isethionate is sodium hydroxide, lye. This is used to make ALL bar soaps, potassium hydroxide is used to make liquid soaps.

Lye is used in soaps because of the chemical saponification that takes place after soap is made.  After making soap it needs to cure for a minimun of 4 weeks. During that time the oils and lye undergo a chemical change called saponification. After the curing time the lye is no longer lye and the oils are no longer oils it becomes soap.