It's there to allow notes to get all descendants, even if they're not roots. I'm not sure if anyone uses it though.
the use of mentions? coracle does it too it's stupid, it should be a third type, so you have root, the OP, and then you have reply, the direct parent, and then maybe you can have ancestors or something, and they should be `e` tags and yeah then you can find all of them but mentioning doesn't do nothin