“It said: ‘Write what you see into a book and send it to the seven churches; to Ephesus, to Smyrna, to Pergamum, to Thyatira, to Sardis, to Philadelphia, and to Laodicia.’”
Here we have John’s specific task. One thing of interest is the contrast between the specific call of John and that of the Old Testament prophets. When God calls to them, he calls them to speak (Isaiah 6:9, Jeremiah 1:9, Ezekiel 3:1, Hosea 2:1, etc…). John is called to write. In fact, Nahum is the only Old Testament prophet whose writings are introduced as a book (Nahum 1:1).
In the case of Revelation, Jesus is the one doing the speaking, as he is the true prophet. John, as his servant, is given the commission to write that which has been spoken for the edification of the church. Like the faithful servants of the Old Testament prophets, John faithfully transcribes that which Jesus is relaying to him.
It is also worth noting that the churches are listed in order that the letter would probably be delivered. Patmos was 50 miles off the coast of Ephesus (it was actually in the domain of Miletus, another Asian city, but one where we have no record of a first century church). It would be read in Ephesus and copied for their own use and then transferred to the next church on the list. The cities are listed in clockwise order as you would travel through the Roman region of Asia along primary thoroughfares.
There is evidence of a second century church in Miletus, though. In Acts 20: 17-38, Paul meets with the Ephesian Elders in Miletus, but there is no reference to there being a church in that city at the time. In 2 Timothy 4:20, Paul relays that Trophimus was left in Miletus because he was sick, perhaps that is the beginning of a church plant. There are no other references to a potential church in the city.