What is main objective of username of user doctype and there are common field between employee and user doctype

I would like to know the main purpose of user name field in user doctype and when it will be useful where use can login only through email id and password.

When we are creating a employee and would like to provide a user id so the employee may login in for applying his leave application ,in this case there are lot of common field between employee and user doctype like use first name,last name, dob etc.
can we have a simple form of employee and user doctype in new erpnext version

User is not necessarily employee but employee is user. User can be your sales partner or may be someone else.
I understand entering same info twice is hectic, to eliminate this we have provided a button “Create User” in Employee, which will make it easier to create user from Employee.

thanks for your quick response.
i would like to share that create user button not working properly even after provide preferred email.
In this regard another issue found i.e there is nothing showing to provide company email in the block of preferred email.

You need to select the Preferred Contact Email and enter its value, it you selected Company Email then enter Company Email in Job Profile section or else Personal Email in Contact section.