So my biggest question is, what can I use this for? I get most languages start as fun hobby projects, but there must be some thought behind this concept right?
Some people really like the idea of concatenative languages. I think this is probably reasonably viewed as "research into making those actually usable", in addition to a fun hobby project.
Here's a blog post that tries to motivate them: https://evincarofautumn.blogspot.com/2012/02/why-concatenati...
Here's another interesting recent attempt: https://www.dawn-lang.org/posts/introducing-dawn-(part-1)/