Jesus was sending the twelve disciples on a practical mission to proclaim to the people of Israel that they should repent (Mark 6:12). He called the Twelve to be fishers of men. The reason they were called as disciples was:
that they might be with Jesus to learn from him,
that Jesus might send them out to preach,
and that they might be given authority to cast out demons (Mark 3:14–15).
They had learned from Jesus, saw him do the work of the kingdom, and heard him preach. Up until this point they were mere spectators and helpers but had not been sent out to preach.1
With this mission they would be given the opportunity to put to practice what they had seen and learned from Jesus. The uniqueness of this mission laid in the fact that they could gain practical experience while Jesus was still on earth with them. He was still there to guide and teach them for their ultimate mission to the ends of the earth (Matthew 28:19–20), after his ascension to heaven.
7 And he called the twelve and began to send them out two by two, and gave them authority over the unclean spirits.