Here is an explicit warning: that Christ will come as a thief. In the midst of disaster upon disaster and global war, some in God's church will be surprised by it. It seems ironic how that could happen, but it is apparently going to happen that way.
Here also is a conditional promise: Those who watch and keep their garments will be blessed.
Revelation 16 does not just reveal prophetic information about the future like some type of crystal ball. No, the prophecy is capped with a command to act: to "watch" and to "keep." Choosing not to remain vigilant, choosing not to guard our spiritual condition from atrophy, we can become complacent. We can become neglectful. Our obedience to the commands to watch and to keep is what is truly important to this particular scripture—not a full understanding of every nuance of this chapter.
In fact, what God wants to see—and in fact, expects to see—is our obedience in faith to the commands of this passage notwithstanding our lack of understanding of the details. In this sense, the blessing promised in verse 15 comes in spite of our full understanding of this prophecy, or lack thereof. Knowledge is not a prerequisite to receiving the blessing. Obedience is.
We believe God's word of prophecy, though we may not always necessarily understand it. Nevertheless, God wants the prophecy to motivate us to obedience, and our obedience will bring a blessing with it.