Post-climax, the denouement serves to wrap up your story. Where are your characters now? How are they doing? What's next for them?
The denouement answers any remaining questions. It provides closure.
Because readers don't want to be left hanging. They want to know that the characters are (or will be) okay after the book closes.
The exception to the rule?
If you're writing a series. In this case, the story should be wrapped up, but not too wrapped up.
Either way, the denouement should grow organically out of who your characters are, the conflict they faced, and the obstacles they overcame.
Be Brilliant!
~Katie~