The main purpose is to be able to unload a KMS driver. It's at least useful to me while working on the DRM stack, because it saves me a reboot :)
The patch introduces a new vt_driver callback, vd_reset_t, which is called after the old driver is stopped (old = fbd) and before the new driver is initialized (new = vt_vga). The callback is really implemented in drm_fb_helper.c where vga_pci_repost() is called (a new public function in vga_pci) to restore the VGA state.
Currently, the DRM code in HEAD seems to turn off the screen during shutdown, and re-POSTing the card isn't enough to turn it back on. The problem doesn't occur with my DRM 3.8 development branch (the console comes back with vt_vga and everything works).