Repeat is a crucial step of shampooing your hair.
Most of your shampoo suggest you rinse and repeat. That is not a lie from the company. Here's the reason:
For healthy hair, it is better to wash them less often.
In order to wash them less often, you have to ensure your hair is thoroughly clean especially the scalp. The first shampoo gets rid of 50% of build-ups and oil. The second shampoo is where cleaning really happens, so it lathers better. If you shampoo only once your hair might feel dirty again the second day. If you rinse and repeat. your hair will last for three days before another wash.
Again, this is not a rule for everyone to follow.
Some people like to shampoo their hair everyday. Which is perfectly fine but you should wash just once with a super gentle formula.
For naturally curly hair, you can stick with washing hair once because the extra oil will make hair less frizzy and easier to manage.
However, for the rest of us the rinse-and-repeat process is crucial. It cleans the scalp, exfoliate the skin and normalizes the pH. More importantly, it prevents oil from clogging the hair follicle. Clog hair follicle will stop your hair growth and as a result hair loss does occur.