The employer needs to know the complete address of the employee for a number of reasons. One is for payroll purposes. Setting up an account for a person requires him to provide his address. Another reason is that if something wrong happens, they would know where to locate him or his family. The company also needs an employee's address so that they can be certain of his identity. And another reason would be for mailing of anything necessary purposes.
A template for an employee form could also contain a portion asking for the employee's office extension number. This would probably be part of the form especially in big companies where an employee may be reached by another in a different branch office through phone.