John is writing to those that he doesn’t know. He is making known to them that he is an eyewitness of those things done and said by Jesus Christ.
Throughout this book, he will repeatedly go back to what Christ said to the disciples and put it in this letter. It is almost as though this letter were sent as a forward to the Gospel of John. He wants those who receive this to have the same fellowship that he and his companions have with the Father and the Son.
Then Who is confessing sins and why?