Traditional Castile soap is made entirely from a vegetable oil (was always olive oil, in the past) that is mixed with just enough lye to fully convert the oil into soap, with no lye left over. That's the basics of all plain soap--saponified fat. Since Castile soap (unlike many commercial soaps) is made with no lard or other animal fats, it is a frequent choice of members of various religious groups who observe dietary constraints regarding animals. It also makes vegans happy.
I have used it for decades simply because it cleans, and doesn't leave a slimy "moisturizer" film or perfume on my skin. Ivory bar soap is not quite as aggressive as Kirk's Castile, and is better suited to my already quite dry hands. Kirk's Castile is a wonderful shampoo as well. No softeners, stiffeners, etc. Your hair squeaks once it's rinsed.
There aren't more than a couple of commercial brands of Castile bar soap, since most people are afraid they might smell like a human. Often, boutique shops will carry artisanal Castile soaps made from the milk fat of various animals--goats, sheep, camels, llamas, yaks, etc.--but it's not really Castile soap.
Bob