Washing your hair keep the dirt and prevent bacteria grows.
When washing your hair, your scalp will be massaged at the same time to improve blood circulation that will help the grow of your hair.
However, the chlorine in the water will damage the growth of hair. If you use a good shampoo and conditioner with filter to the water, your hair will grow normally half an inch a month. You also need to have good nutrition and calcium to your body.
There are lots of knowledge and you only need some basic ones. Do not believe those ignorant beliefs.
Hope you can read my English as I cannot type Chinese.