Paul was finished with his second missionary journey and on his journey back to his home church in Antioch. But Luke takes us back to a new character, Apollos.
Apollos is a common name in the New Testament but a relatively minor character in Acts. It's puzzling why Luke would take the time to introduce us to him without him ever being mentioned again in Acts.
Eventually, Luke's reasons come through. For as prominent as Paul is in Acts, this is not Paul's story. This is the story of how Jesus is continuing his work through the Spirit. Luke leaves no room for celebrities in his story.