First: Hurd isn't a new operating system. It's a decades old project from last millennium.

And then: Doing research in operating systems serves a lot of purposes. For some it's just fun. For some it's experimenting which may lead to ideas which may be incorporated into other OSs later, where eit is a lot simpler to do in a small kernel. For some it is an attempt to take over the world, few of those will, but maybe one might. At least for a small part of the world.