For example, see https://www.recursive.design/, which is a font that's highly customizable. It allows modifying all of these variables in a scale:
This is particularly interesting for Gio, because if one wants to ship an entirely self-contained app, it's a good idea to ship fonts as well. But if one wants a monospace font for code, a sans font for English, and a cartoon-ish font for something else, you likely need to ship three fonts.
With a variable font, you could ship a single one and then tweak the parameters in each of the UI components. Note that I have very little experience with font rendering, so I don't know if this would be a big refactor, or simply exposing this font as a package.
HN discussion: https://news.ycombinator.com/item?id=23929151