Same. It was only when I got to reading the user manual that I see what I feel could be the first sentence of their website

Nickel is a generic configuration language.

Its purpose is to automate the generation of static configuration files - think JSON, YAML, XML, or your favorite data representation language - that are then fed to another system. It is designed to have a simple, well-understood core: it is in essence JSON with functions.

https://nickel-lang.org/user-manual/introduction/