It's the most straightforward way to ensure the directory structure remains a directed acyclic graph.
As an advanced feature there's plenty of room for abuse and footgun problems, but it's still occasionally, maybe even rarely, the correct tool for the job.
Besides, won't this walk and copy the entire directory subtree? (i.e., CoW only applied to the files themselves, not just directory entries.)
I'm saying I'd like to just have a hard link to the directory itself; if the directory is effectively a list of inode pointers, I just want to add another pointer to that list of pointers. Then, if I create or modify a new file in some deep nested directory in either copy, it will be instantly available in both or more locations without any metadata changes on disk to more than one copy of the metadata itself. This would be interesting for certain use cases.