When employment starts, so does the employer_employee relationship. This type of relationship is defined by the kind of contract the employee has in the company. It revolves around the things they talk about and disclose to each other before the start of the employment. And before such employment, it is necessary that the company has certain basic information about the employee. This is basically for accountability purposes and for the sake of having a complete and uniform record of all employees.
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.