"Why do people have to put a Child number after songs?"
Nothing to do with cleverness; it's a reference number for people to seek out other versions should they wish to. The song is known by literally dozens, if not hundreds of titles; (Scarborough Fair, Whittingham Fair, Jenifer, Gentle and Rosemary.....) Child made it easier to reference them.
Steve Roud has done an excellent job in giving all folk-songs a number; nothing clever about that either, but I'd be in trouble, living where I do, if he hadn't.
Jim Carroll