First, in revelation God refers to Himself as Father.
Second, it has to do with His nature as creator. The gnostics would use the image of God as mother to advance a semi-pantheistic view. Because God in relation to his creation is completely seperate just as a father does not bear his children. However, the power of God affects creation just as the power (sperm) of a father affects the fertal womb. In other words it helps us keep in balance the reality of Gods emanence and immanence in creation.