The choosing and training of the men to whom he was to entrust his work was an extremely important part of Jesus' earthly mission. Jesus' primary purpose in coming into the world was to die as the Lamb of God in expiation (compensate) for human sin and to rise from the dead to bring eternal life to mankind. But his Life, Death and Resurrection would be useless to the world unless the world knows about it. If the men to whom he entrusted his work should fail him, then his coming to earth would have been in vain.
The first sending out of the Twelve was a part of their training, intended possibly to give them practice work, and was a part of Jesus' method of advertising to the nation that the Messiah had arrived. There were no newspapers. The only means of spreading the news was by word of mouth. Later the Seventy were sent out for the same purpose. These men authenticated their message by special miracles, not only to attract attention, but also to indicate to the nation the extraordinary nature of the one whom they proclaimed.
Their training was not an easy task, for they were being trained for a work utterly different from what they thought they were being trained for. They began to follow him as politicians with no thought whatever of becoming preachers, as they turned out to be. They were expecting that, as the Messiah, he would establish a political world empire, of which they would be the administrators.
His method of changing their minds about the work that He and they were to do was, first, to present himself to them in all the fullness of his divine glory, so that, no matter how differently he talked and acted from the way they expected Him to talk and act, they would still believe that he was the Messiah. That is one of the reasons he wrought miracles, and that he was transfigured.
Then, along with that, he spoke in parables, veiled sayings, to give them the impression that he did not always mean exactly what he appeared to mean. This kept them for a while in a state of wonderment. If he had told them plainly at the start, they might not have been interested in following him at all.
When at last he told them that he was going to be crucified, instead of erecting a throne, it stunned them. But they still persisted in thinking it was only a parable. Even at the Last Supper, their minds were still on which one of them was to have the greatest office.
Not till after his Resurrection and the descent of the Holy Spirit did they at last come to understand that it was to be a kingdom in which Jesus would reign in the hearts of men, and that their part would be simply TELLING THE STORY OF JESUS. That is all. The Story would do its own work. If men know about Jesus they will love him.
Reference: Halley's Bible Handbook- Zondervan Pub. House, Grand Rapids, Mich. Jn. 1:35-51, Mk. 1:16-20 Lk. 6:12-16, Matt. 28:16-20.
The King James Version of The Holy Bible