Originally there were only Bishops which is why St. Paul only addresses Bishops and Deacons with St. Timothy in Scripture. As the Church expanded growing in size the Bishops needed assistance but they wanted to control whom would be ordained so a distinction was made between Bishops (Chief Priests) overseeing the duties of their Priests whom were given the authority to preach His Gospel and administer the Sacraments. The word "co-worker" is appropriate being they they are their peers the only exception being that a Priest can not ordain.
