I think anything you would normally write a schema for, and edit by hand. It does actually come with a schema language too, embedded in KDL, based on JSON-schema. I think it would be a great syntax for something like CSL (Citation Style Language, something I work with); not a document, no actual content, but free-flowing, recursive, needs a schema, and is edited by non-programmers. There's a lot of talk in CSL-land about graphical editors because XML is just too hard, but graphical editors are also too much effort to build. Someone once suggested based on general facts (lighter, etc) about JSON that CSL should switch input language to JSON -- that would have been a disaster, JSON is terrible for this. (And the CSL project wouldn't change its spec dramatically and unnecessarily.) But KDL seems good for that kind of thing.

Unfortunately I don't quite see it as a replacement for Jsonnet (or Dhall, etc) in terms of writing extremely laborious Kubernetes/Terraform/etc config by hand. It doesn't seem very easy to embed a functional KDL-generating language in KDL. I sat down and tried to imagine what this would be like and couldn't make it connect. Maybe someone else can give this a shot, there are devops folks out there that need saving from YAML. Part of the problem for KDL as a replacement for these is that its mapping to JSON/YAML is a bit awkward, whereas XML is actually pretty close. That's presumably why Kat called it their Document Language. So I expect if it catches on it will be in greenfield projects or XML revamps.