

Soap and conditioner have opposite and incompatible roles. Soap strips away dirt and oils. Conditioner replaces the oils stripped away by soap that your hair needs, to maintain health.
You don’t really need separate shampoo and body wash, you could wash your hair with standard bar soap. You do need separate conditioner.
Shampoo+conditioner combined together can either clean or apply oils to the hair, but chemically, it cannot do both, despite marketing.









What about leather, could it benefit from applying a layer of oil? Or wood? Or rubber? Or cast iron? Living tissue is not the only thing physically affected by moisture levels.
Oil seals in moisture to prevent hair from drying out, which, obviously, will change it’s texture and elasticity. The skin on your head naturally produces grease to keep your hair moisturized, but when you wash with soap, it strips that grease out. If you shower regularly, you need to replace it.
I used the word “health” because it is commonly used to describe hair texture. You won’t be medically unhealthy if you don’t use conditioner, but your hair will probably be dry as shit and frizzy.