en.javascript.info/5-network/04-fetch-crossorigin/1-do-we-need-origin/solution.md

We need Origin, because sometimes Referer is absent. For instance, when we fetch HTTP-page from HTTPS (access less secure from more secure), then there's no Referer.

The Content Security Policy may forbid sending a Referer.

As we'll see, fetch also has options that prevent sending the Referer and even allow to change it (within the same site).

By specification, Referer is an optional HTTP-header.

Exactly because Referer is unreliable, Origin was invented. The browser guarantees correct Origin for cross-origin requests.