Luke's primary purpose in Acts is to describe how the gospel was spread, and how the church grew. However, many other things were also going on, which affected the church. This prophecy by Agabus is an example. At this time, there was no 'New Testament' such as we have to instruct the church. So, Jesus appointed apostles to define what should be taught. In addition, the church recognised that God had gifted some people with the ability to be prophets, evangelists, pastors or teachers (Ephesians 4:11-13).