I don't know the scope of "nothing" in your statement, but shampoo does help remove dirt and oil, in a way that washing with water only cannot achieve, which is the number one goal of using shampoo for most people.
This is verifiable by observing and touching hair of other people's hair before and after shower, which eliminates the possibility of shampoo manufacturers secretly altering what you perceive with your fingers.
I don't know the scope of "nothing" in your statement, but shampoo does help remove dirt and oil, in a way that washing with water only cannot achieve, which is the number one goal of using shampoo for most people.
This is verifiable by observing and touching hair of other people's hair before and after shower, which eliminates the possibility of shampoo manufacturers secretly altering what you perceive with your fingers.