I know some of you are going to say "you already made this thread" but that thread had a different OP and questioning, this is pretty straight forward (no melting scalps).
So why do we use shampoo?
Salt and water cant provide full nourishment to the scalp and hair. Shampoos were made to gently clean all the gunk and impurities from your scalp and head. Soap is more damaging to the hair. It is a proven fact that shampoo help improves the health of the hair and provides full nourishment to the hair fibres.
If you stop using shampoo for a longer period of time, your scalp will start to adjust and secrete less oil.