Extremely simple static site generation (like Grandma used to bake in the 90s web). Either by hand with minimalist HTML or just taking the output directly from almost any Markdown/reStructuredText/Asciidoc/etc processor and generally only needing to add a single LINK tag at the top to one of these CSS files.

Places where you want to auto-generate html reports or do simple data processing into html and not fuss over basic inline styles.

I have a few internal tools that use https://newcss.net/ and it is just a nice tool to have in your quiver.

Easy to toss on content like markdown or straight out of a CMS and get nice looking pages vs. the default browser styles (that look like 1996).

you can switch between them without altering the html