They've checked in my code in their own repository, using an automated tool that keeps track of its origin so they can still check for updates. (The build tool knows to check this directory before trying to pull in dependencies from elsewhere)

One benefit to them is that their build won't break if I decide to delete that specific repository (see also: the left-pad incident) or do silly things with version tags (deleting versions, or re-tagging a different commit with the same version number, that sort of thing).

But more relevantly for this thread, it also means that if I release a new version and they upgrade to it, the PR on their repository won't just be incrementing a version number in go.mod and adding an unreadable hash to go.sum: the diff will show all the changes I've made since the version they previously used.