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.
The first benefit to consider is the ability to clearly delineate the roles and responsibilities of both the company and employee in a single document. With a form template, this process can be quite easy and quick, even as simple as filling in some blanks and signing. A good form will have some empty lines to write