Lots of sites see an empty referrer and send you to their main page or marketing page. Which means you can't get anywhere else on their site without a valid referrer. They consider it a form of "hotlink" protection.

(I'm not saying I agree with it, just that it exists.)

Fair and valid answer to my wording. Rewritten for what I meant to ask: "Why set referrer to the base of the destination origin instead of something like Referrer-Policy: strict-origin?". I.e. remove it completely for cross-origin instead of always making up that you came from the destination.

Though what you mention does beg the question "is there really much privacy gain in that over using Referrer-Policy: same-origin and having referrer based pages work right?" I suppose so if you're randomizing your identity in an untrackable way for each connection it could be attractive... though I think that'd trigger being suspected as a bot far before the lack of proper same origin info :p.