[libvirt] Node devices?

After staring at the code implementing node devices in libvirt, I have decided that they're really neat, but I have no idea what they are for or how to use them. Reading the API docs and searching libvir-list didn't help much, and the user documentation (http://libvirt.org/archnode.html and http://libvirt.org/formatnode.html) is a bit, err, high-level. Can someone enlighten me? --Ed