When you're writing a Go application and want just a few things out of a web server, something like this is a nice demonstration on how to do that with a few lines in your Go program. Yes, you could use Caddy, or you could even import it and use it as a library; but if you prefer something like this, there you have it.
I'm guessing this is a demonstration. Perhaps it's showing you all the things you can do with Go's in-process webserver library instead of an external webserver like Caddy?