Natural leather/hide requires an 'internal' lubricant/moisturiser to remain supple. Using any detergent and water will progressively remove those natural oils until it becomes stiff and unpliable. The use of 'saddle soap' both cleans the surface and maintains pliability by providing a moisturiser (typically glycerine) and another natural oily material called neatsfoot oil. Neatsfoot oil is made from rendered down cattle bones, which gives it the characteristic smell noted by
trikey. It would be a mistake to clean leather long term using a simple detergent bar alone. Chamois leather is a good way to demonstrate how the internal oils get lost. When you buy one, it's soft and pliable. but a couple of uses will remove the moisturiser and when dried out, convert it to a lump of crushed cardboard. Neatsfoot oil in a leather seat is likely over time to transfer to your clothes, so one of the more sophisticated dressings would be better. Synthetic/non-smelly neatsfoot oil alternatives are used in these. Neatsfoot oil:
https://en.wikipedia.org/wiki/Neatsfoot_oil
TC