St Joseph was proclaimed Protector of the Holy Church since he so willingly and honorably defended Our Blessed Mother and Jesus.  He was entrusted by God to take care of the human nature, growth, and development of Jesus. The divine nature of Jesus did not require anything from St Joseph but human nature required his fatherhood.

St Joseph stands ready to shelter you in the safety of the Sacraments and teachings of the Catholic Church, clothe you with virtue, educate you in the interior life, protect you and correct you, should you go astray.
                                                                     From ‘The Consecration to St Joseph” by Fr Donald Calloway