For one thing, massive exposure: Have a look at the Google Fonts stats page.
Google Fonts are used on websites all over the world and the most popular accumulate literally trillions of views.
Google Fonts can also be used on the desktop, so if your work is selected, it is likely to be seen and used by a lot of people in a wide variety of different design contexts and applications.
Note that Google Fonts is a free service, and there are no direct financial benefits for designers whose work is included in their directory.
For more information on the Google Fonts project, please consult the Google Fonts website.
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