Ark Royal and other carriers also carried armour; Illustrious's distinctive feature was the inclusion of her hangar within the protected structure. British planners expected some attacks to get through, so they wanted damage to remain contained and repairable with limited facilities. That mattered especially in the Pacific, far from Britain's main industrial base. A carrier forced to return to Britain for repairs could spend additional months away from operations simply travelling to and from the repair yard.
Chapter · 0:21-7:34
Why Illustrious protected her hangar
Illustrious's armour scheme was meant to contain damage and preserve a carrier that might be far from a major repair base.