Yeah, the Hallows seem...not obviously helpful, and it isn't clear why anyone thought they were a good idea, but if they helped fix the timeline (or get a better one?) they start to make sense.
Ma Gui made an offhand remark about being surprised about their function/"So the legends were true!" so ... I have no idea who actually made them, or who made the blueprints. (Maybe future discussion on the Hallows will help?) I guess they knew there were time-related properties, and Fu You wanted to affect the timeline/maybe desired to undo the meteor collision?