I guess the purpose is to be able to put different UUID generation schemes into separate kind-of "namespaces". That way, a UUIDv4 (random) will never collide w

I guess the purpose is to be able to put different UUID generation schemes into separate kind-of "namespaces". That way, a UUIDv4 (random) will never collide with a UUIDv1 (timestamp+mac+...), which again will never collide with UUIDv5 (SHA-1). So if one of the UUID generation schemes is flawed, these are easily filtered out, and moreover, will never interfere with any other UUID version/scheme.