I’ve read in an article that one’s hair produces natural oils that actually help ‘clean’ the hair, so if you were ever marooned on a deserted island, your hair wouldn’t get as dirty as you think it might. Of course, compared to living among civilization, walking around with a week’s worth of unwashed hair is pretty bad, even with all the natural oils.
So thank God for shampoo! Even a little goes a long way to wash out all the dirt that gets trapped between hairs. Conditioner is an added bonus, too!