If I remember correctly, the PO position descended from the "Service Coordinator" who was assigned to each congregation (or, back then, "company") during the Rutherford era. The SC was a Society man who functioned among elders who had been elected by the congregation. This gave the central organization more control than they had had previously and they had an active agent in each congregation.
When the "theocratic arrangement" came in during the late 1930's and the electing of elders was done away with in favor of central, "theocratic" appointment by the Society, the Service Coordinator became the "Company Servant." In later years, the term for "company" was changed to "congregation" (undoubtedly to avoid sounding quite so much like a commercial enterprise), and the "Company Servant" became the "Congregation Servant," or, more informally, simply the "Overseer" of the congregation.
With the 1970's came the elder arrangement, and a new distinction was drawn between elders/overseers and "ministerial servants" (previously, in almost Baptist fashion, the Congregation Servant was viewed as the only elder/overseer, and all others appointed to leadership positions were considered "servants"). Formerly, there had been positions like "Assistant Congregation Servant," "Bible Study Servant," "Watchtower Study Servant," etc. Also, positions such as "Literature Servant," "Accounts Servant" and "Magazine/Territory Servant" had been specifically appointed by the Society. Under the new arrangement, the teaching positions became elder positions (originally Presiding Overseer, Field Overseer and Bible Study Overseer along with Watchtower Study Overseer, Theocratic Ministry School Overseer and most Book Study conductors) and the non-teaching positions (literature, accounts, magazines, territory etc.) were assigned locally to "ministerial servants." Additionally, the elder positions rotated among the elders from year to year.
Later, the rotation was stopped and the positions were appointed permanently. The Field Overseer became the Service Overseer and the Bible Study Overseer position was eliminated in favor of the Congregation Secretary. Now, it seems, they have come full circle and the PO has again become a "coordinator" much as he was back in the Rutherford Era.
And, really, I'm not sure what the point of all of the above was except to illustrate how silly and insignificant changes are seen as Earth-shaking within the JW organization, and how much turmoil the WTS has to create on an ongoing basis over non-issues in order to keep the rank and file preoccupied with such things so that they won't have the time or energy to think about whether what they are doing is actually God's work.