There is no reason for its inclusion other than for aesthetic purposes. Cosmetics companies realised that consumers preferred a thicker shampoo because it seemed more effective, and so they began to add salt to their products.
You may indeed prefer a shampoo with a thick, luscious appearance, but actually salt is an irritant and can damage the hair and scalp when used repeatedly over time. Think of the pain it causes to an open wound - this is due to chemical erosion. It does the same to the outside of skin and hair, only the effect is painless.