I tend to think of variable fonts as a font format in which a single font file is capable of displaying type at near-infinite variations of things like boldness, width, and slantyness. In my experience, that’s a common use case. Just check out many of the interactive demos over at Axis-Praxis:
Make sure to go play around at v-fonts.com as well for loads of variable font demonstrations.
But things like boldness, width, and slantyness and just a few of the attributes that a type designer might want to make controllable. There are no rules that say you have to make boldness a controllable attribute. Literally, anything is possible, and people have been experimenting with that quite a bit.
If you’re interested in variable fonts, we have a whole guide with all the best articles we’ve published on the subject.
Here’s some, ahem, weirder things that variable fonts can do.
A variable font can change its own serifyness
A typical job for a variable font is changing weight/width/slant… but it really can be _anything_.