What are benefits of being bald?
It helps to identify you socially as a monk (as does the uniform clothing), so that people know when they see you, "that person can be identified as a monk".

At the same time it helps to alter, remove or hide your personal identity, e.g. as a person with an individual appearance such as hair color and hair style.

I suppose another practical benefit is that it's also clean and cool (an opposite extreme, for some non-Buddhist sramanas, was to not touch or cut their hair at all, so that it became long and matted).

So far as I know the practice pre-dates Buddhism: MN 26 says,


  So, at a later time, while still young, a black-haired young man endowed with the blessings of youth in the first stage of life — and while my parents, unwilling, were crying with tears streaming down their faces — I shaved off my hair &amp; beard, put on the ochre robe and went forth from the home life into homelessness.