We can learn much about this letter by the way it begins. The structure of the letter sends a strong message about the intention behind the letter. There's no denying the futuristic thrust of much of Revelation. More than any other book in the Bible, it tells of things to come.
But having said that, I don't think the primary purpose of the letter is to engender heated debates about the time of the rapture or the length of the tribulation. This is made clear in the very first verse of the first chapter:Revelation 1:1 - "The revelation of Jesus Christ, which God gave him to show to his servants the things that must soon take place. He made it known by sending his angel to his servant John..."
True, the letter shows us things to come. But even before that is said, we're told this is a revelation of Jesus Christ. Not just a revelation from Jesus Christ - a revelation of Jesus Christ. In other words, this letter isn't just about the return of Jesus Christ in the future. It certainly includes that. But this letter is about the person of Jesus Christ. It's a revelation of His unimaginable glory and majesty and power and promise right now.