The website confused me initially, but it's a service for rendering diagrams. You send your diagram in one of 20-ish formats via the URL or via a POST body, and it returns the graphic. You can install it locally.
Installing it locally seems to require running the backend as a web service (via Docker or locally). The cli tool connects to that. A bit heavy for a tool you use incidentally.
I think it's more for documentation projects where pages might be rendering lots of diagrams in different formats, like Wiki.js, rather than personal use.
This site made me wonder if it's possible to embed PlantUML diagrams into Notion. One quick google search, and a top result referenced using Kroki. Serendipity.
Liking Kroki; I'll probably have a use for that (eg C4).
In return, if anyone just wants a simple place to create a sequence diagram (only) and prefers to do it interactively, then (thanks to js-sequence-diagrams, credit-linked from the site) a while ago I stuck a static site on Netlify: https://sequence-diagrams.netlify.app/
It's a good observation that a lot of people want "diagrams", not separate, incompatible graph layouts, scatter plots, maps, histograms, radar charts, and everything else. Grammar based libraries and tools are starting to fill this need, but still feel low level, and sometimes too much DIY integration. Great problem though and exciting to see progress in understanding practical needs.
Neat.