The short answer is: Because it may ping under load without it.
Basically the engine was designed (e.g., compression ratio) and tuned (e.g., ignition advance) to make optimal power on 91 AKI / 96 RON.
That said, it is worth noting:
1. The owner's manual claims it may ping under load anyway
2. It runs very rich approaching fuel cut; rich mixtures cool the combustion chamber with unburnt fuel, and acts as an anti-detonant.