To make it shiny. The vinegar strips away excess shampoo residue and oils.
I love apple cider vinegar, I have been using it about 2-3 times a week as a hair rinse. My hair is fine so I am even able to get away with the vinegar rinse as my conditioner too! My hair is very shiny and I have noticed that I can wait almost a day longer than normal, no oily roots!
I will use for life!