nevents have the event id already, and they can have the author pubkey there too for more future-proof hinting. Relay hints serve as a good-enough hinting mechanism not only for fetching of that event immediately, but also for associating those relays with the pubkey that authored the mentioned event in a client store -- those associations can later be spreaded around and help other clients that are having trouble fetching data from such pubkey, but they can also update over time.