Webfonts were great when most computers only had a handful of good fonts pre-installed. Thanks to font creation and buying by Apple, Microsoft, Google, and other folks, most computers have good - no, great - fonts installed, and they're a great option if you want to not load a separate font.
No network request, no time to parse a font, no flash of an incorrect font.
System fonts have lots of styles and broad language coverage, unlike many webfonts.
Web apps feel more native when they use system font faces.
-apple-system and BlinkMacSystemFont are aliases for the default fonts on new macOS and iOS computers. In recent version, they alias to the new San Francisco font.