If a user has a complicated set of department security rights it can be helpful to inactivate the user when they no longer need system access.   When a new person who needs the same security as the inactive person is identified the Duplicate User function can be used to copy all the security rights of the inactive user.  This can be an very efficient way to manage security. 

Deleting a user does not delete Audit History.

If the user's security setup is not complicated then the user can be deleted.