They are everywhere. For a good reason as well - typography plays a big role in UI design, and Google provides designers and developers a simple solution by providing popular fonts over their CDN. You might, however, have good reason to serve these from your own server.

Google provides plenty of web services, one of which is Google Fonts. At the same time, using these services is as simple as opening a window (or browser tab, literally speaking). You might have a good reason though not to use these, and instead, provide font files from your own sources.