It's marketing for the origin site. The line of thought is that the author sees significant traffic from xyz.com in the ref query string, and considers advertising or partnering with the origin site.

Honestly, it is quite useful for niche/startup sites. I have been on both ends of conversations that began from seeing these in web analytics (as someone that saw incoming traffic from a site and reached out, and as someone that received contact from a site I linked to) - and both times it ended in a mutually beneficial partnership.

I can understand the privacy argument to some degree, but it provides no more information than the standard Referer header (and if you use analytics like Simple Analytics/Plausible, it is a lot more visible).