There seems to be a reply like this at the top of every font-related thread on HN. I often wonder whether it's coming from designer/developers or from end users.
Web fonts aren't popular just because they're pretty. They continue to be used everywhere because they offer a consistency that's impossible to get otherwise. With a plethora of mobile devices and desktop environments, you can't get a consistent look-and-feel for a site with just installed fonts. For some really basic sites, that's totally okay and even hits a nice quaint aesthetic, but for the kinds of sites that people pay money for, that's not good enough.
Variability in system fonts means that getting consistent paragraph and header widths and spacing is impossible. It means that the text looks too dark sometimes and too light other times. It means that the font appears unreadably small sometimes and obnoxiously large other times. It means having to build a font stack and then keep it updated as preinstalled fonts change. It means spending a lot of time dealing with issues like Helvetica Neue looking okay on MacOS but rendering like ass on somebody else's desktop because they installed a free knock-off of it. It ultimately introduces a lot more complexity and fragility than properly using a locally-hosted web font.
Web fonts aren't popular just because they're pretty. They continue to be used everywhere because they offer a consistency that's impossible to get otherwise. With a plethora of mobile devices and desktop environments, you can't get a consistent look-and-feel for a site with just installed fonts. For some really basic sites, that's totally okay and even hits a nice quaint aesthetic, but for the kinds of sites that people pay money for, that's not good enough.
Variability in system fonts means that getting consistent paragraph and header widths and spacing is impossible. It means that the text looks too dark sometimes and too light other times. It means that the font appears unreadably small sometimes and obnoxiously large other times. It means having to build a font stack and then keep it updated as preinstalled fonts change. It means spending a lot of time dealing with issues like Helvetica Neue looking okay on MacOS but rendering like ass on somebody else's desktop because they installed a free knock-off of it. It ultimately introduces a lot more complexity and fragility than properly using a locally-hosted web font.