Once employed, the employee usually fills out an employee form that he needs to fill out with the information requested. An employee form template would normally look like a form with the name of the company on top, and a title stating New Employee Form.
The social security number will be part of the template. It will provide the company everything it has to know about the employee. If he has an outstanding warrant of arrest from the police, if he is an outstanding debt, what responsibility he has, what violations he did, etc.