In the scenario, torpedo bombers need fighter cover to avoid being sent into an attack unprotected. Keeping a fighter flight ready on deck preserves a choice between escorting that strike and reinforcing the combat air patrol. The reserve can also replace patrol aircraft that need to recover. Committing every available flight to CAP would remove those options until aircraft returned. These choices illustrate how a carrier must divide its fighters between immediate defence and the next mission.
Notable Point · 55:55-58:23
Why a carrier keeps fighters ready on deck
Keeping a fighter flight ready preserves the choice to reinforce combat air patrol or escort an outgoing strike.