Y the time we get to Matthew 10, it is apparent that Jesus had collected all 12 of His leading disciples. Here it is recorded that He imputed to them special spiritual power and authority. This gift continued with them throughout the remainder of their lives. It was a way for God to get people's attention for them, and a way to validate their message. The same kinds of manifestations of God’s miraculous power are no longer necessary (because the New Testament has been completed and confirmed). God is the same, but in this era, there are no apostles and no necessity of sign-gifts. It doesn’t mean the power is gone, but the immediate purpose is.
Now, "The Twelve" went on a mission trip to their fellow Jews. The gospel was sent first and foremost to God's elect nation. We know from other passages of Scripture that the gospel also has a broader audience; a worldwide scope; a kind of "every creature" aim (1st John 2:2, Romans 1:16 & 10:17-18, John 12:32 & 3:16, Revelation 5:9 & Mark 16:15). But, at this point the invitation was sent to Israel. God's ideal plan looked like this: the Jews would believe, and then THEY would proselytize the rest of the human population. We know that the Jews did not cooperate – at least, not yet.