For starters, you can tell who has unlocked the gate just looking at the lock, or deducing it by the remaining ones. This also prevents the gatekeeper from switching out the lock and rendering the 8 identical keys useless. Locksmiths can master-key a lock, but that wouldn't allow 8 unique keys. And even if you could get more than 3 unique keys (more pickable as you add keys), there's no real advantage other than it's a pain in the ass to re-key., but you lose out on knowing who opened the gate.