Why was it necessary for Joseph Smith and Oliver Cowdery to be ordained as elders in the Church on April 6, 1830, when they already had received the Melchizedek Priesthood under the hands of Peter, James and John?
(Answer on page 13)
Here's answer to week's question
(From page 2)
When Joseph and Oliver were ordained to the priesthood by the three apostles, there was not yet an organized Church of Jesus Christ on the earth and no need of Church officers, such as elders, priests, teachers or deacons. Consequently, when the Church was organized on April 6, 1830, it was necessary to ordain Joseph and Oliver to the office of elder.
Source: James E. Talmage, Jesus the Christ, p. 778