>>What was the practical purpose of those men the Apostles chose as leaders?<<
Are you talking about the “fellow servants” who they left as the designated teachers at each of the churches?
I sees no reference to “fellow servants” but rather elders and overseers.
I say this only in reference to leadership, for I do believe the first work of the church is service.
Oh, and by whose or what authority did they designate teachers?
By your own words, you admit that the Apostles provided both successors and a manner in which to chose them.