Revelation is in a sense the book we could do without, because it is a summary of the Bible. It doesn't really introduce anything new that hasn't been written earlier. What is new is the style in which it is written. It's like having a newspaper article and a cartoon that goes with it. You could do without the cartoon, it says the same thing, but it says it in a different way, and your appreciation of the article is enhanced by it.
Revelation is simply a visual presentation of the gospel.
He testifies to everything he saw - that is, the word of God and the testimony of Jesus Christ. He saw the word of God and the testimony of Jesus. In other words, what John saw was the gospel. But it was in audio-visual format. Revelation is simply the word of God, the gospel about Jesus Christ.