When God shows us something, we have an obligation and the privilege to share it with others. This is exactly what we see happening in Revelation. John has just laid out that this was a revelation from God given to Christ, which Christ sent with an angel to John. John says of himself in Revelation 1:2,
who bore witness to the word of God and to the testimony of Jesus Christ, even to all that he saw.
This revelation was the very "word of God," and it was testified to John by Jesus Christ. In the following chapters, we'll see exactly what it was that John "saw." But the point of this post is simply to point out that John is bearing witness to what He saw and how that's absolutely the right thing to do.
Certainly, part of the obligation of writing this all down is that it's what God had intended for this revelation. It wasn't just for John; it was for Christ's church for all ages. But even if John wasn't instructed to write it down and share it, why wouldn't he? He had just seen amazing things from God; why wouldn't he share them with others?
You see, that's essentially what discipleship and evangelism are. We overcomplicate it sometimes. But in reality, discipleship and evangelism are when one person shares with another what God has shown them in His word. And why would we not do so? God's word transforms people. It brings them out of darkness and into light, and for those who are already in the light, it just draws them even closer to God. Why would we not say what we see? Why would we not share with others what God has shown us in His word?
It's the logical response for those who truly believe what God says. It's an obligation, in the sense that it would be quite selfish for us to keep it to ourselves. And it's a privilege because our time to share what we have seen on this earth is limited. So, say what you see.