System font stacks got hot about a year ago, no doubt influenced by Mark Otto's work putting them live on GitHub.
The why, to me, feels like (1) yay performance and (2) the site looks like the rest of the operating system. But to Mark:
Helvetica was created in 1957 when the personal computer was a pipe dream. Arial was created in 1982 and is available on 95% of computers across the web. Millions, if not billions, of web pages currently use this severely dated font stack to serve much younger content to much younger browsers and devices.
As display quality improves, so too must our use of those displays. System fonts like Apple’s San Francisco and Microsoft’s Segoe aim to do just that, taking advantage of retina screens, dynamic kerning, additional font-weights, and improved readability. If operating systems can take advantage of these changes, so too can our CSS.