> Glyphs are taken from the default user interface font on a given platform. Because typographic traditions vary widely across the world, this generic is provided for typefaces that don't map cleanly into the other generics.
So it might have been used to make the snippet "truly" universal for the developer using it. That said, I believe it would be better to choose the font family based on the content.
Even if a viewer is from a place where Serif/San-Serif/etc. doesn't make sense, they are still viewing the same content as anyone else.
I still don't get it. You don't know what users select as UI font, how can it be universal? On the other hand, you do know what font they like for text, but decide to ignore it and serve them text rendered in what they selected for buttons!
You are correct not to get it. system-ui has been abused from the very start as a proxy for prettier default fonts, but it has semantics, and those semantics make it unsuitable for use as a body font. https://infinnie.github.io/blog/2017/systemui.html gives a simple demonstration of how it can go wrong.
> font-family: system-ui
Why render texts with the font that the reader has selected for a different function, ignoring the one that she has selected for texts?