As per field level of of Show Employees as Contacts under Setup > Company > General Preferences: If this box is checked, a Contact record is created for each Employee record users create.
By setting the Show Employees as Contacts= T, this creates a direct link with respect to the Contact and Employee record.
If users change the Email address on the contact record, it also affects the Email address on the Employee record or vice versa since both entities now share the same information. This actually also applies to any other change done on one record. The same change happens to the linked record.