Also, the common argument for this technique that CDN lets you cache content so that users don't have to re download it every time. But I think it could also be done at the HTML level by precising a hash value of the file that we want to integrate (I stopped web dev for a few years I don't know if that has been added to HTML ?)
The property is used to ensure the CDN is serving legit content that has not been tampered with.
/* latin */ @font-face { font-family: 'Tangerine'; font-style: normal; font-weight: 400; src: local('Tangerine Regular'), local('Tangerine-Regular'), url(https://fonts.gstatic.com/s/tangerine/v9/IurY6Y5j_oScZZ784Ox...) format('woff2'); unicode-range: U+0000-(...) }