Intend and purpose? I mean, it sounds so very subjective with no link to research that it matters that much assuming it’s good enough in the first place.

Sure, DIN or the typefaces used at the airport to guide pilots are extremely legible, and less ambiguous in their context. But the difference between Fraktur and SF Pro is much bigger than between SF Pro and DIN for example. Plus, being on hundreds of millions of devices made it common so people got used to it.

Reminds me of the difference between Kai and Song font types in Chinese.

Is there any research with a graph of legibility over fonts? Any idea where the point of diminishing return is for different activities?

I do like a good programming font, but if I’m honest with myself, as long as the foundation is right, most differences are to accommodate one’s taste. I’ve yet to found a case where there was a bug because my font threw me off, or something.

Side note: I recall the first time I read digits written in the English style, and got tripped because the 7 looks like a 1 in my culture, because we all add a little horizontal bar in the middle. You can used to it, but on hand written notes, nobody would write an English 7, yet virtually all computer fonts only offer one version of it (Berkeley Mono offers both, in a mono space font!)