it's an option to upsell higher category rooms. usually at a lower price than booking the room outright. eg: I booked my base room at SGD270 + NOR1 SGD150 = SGD420/night. Booking via Hilton would have been ~SGD600.

but you'd only know what the NOR1 rate is AFTER making the original booking, so while it's cheaper most of the time, check the Hilton website for the direct rate to be sure you're not paying more for nothing.

there's usually a link in the original Hilton reservation confirmation email and in the Hilton app (Request Upgrade button).