Probably easier to think of it more as a fast, native-backed library for a JS runtime like Node to build Markdown processors/pipelines or even just do parsing. (Though it does have a WASM build for browser/edge use.)

Processing markdown into html is a significant task for many blogs, sites, and apps. Satteri moves that processing into rust for speed gains.

Now used in Astro 7: https://astro.build/blog/astro-7/#markdown--mdx-in-rust