. . . thereby hangs a tale, as they say.
Most hymn tunes have their own name, because different words can be sung to the same tune (and the same words to different tunes!) In the case of Sine Nomine, the tune and the words are closely married because of Vaughn Williams' somewhat complex setting, but that isn't always the case.
BTW, "Sine nomine" means "without name," usually indicates that the publisher's name is not attached. But in this case the tune's name is "No Name." Sort of like Odysseus telling the Cyclops that his name is "Nobody," or the tinker woman who was under bond never to tell her name, so she was "Ban Gan'Ainm" or "Woman No-Name".