A capital ship could buy a carrier room to strike

Why pair a capital ship with a faster carrier if the carrier can simply withdraw?

The heavy ship and cruisers can form a dangerous rearguard, deterring pursuit and buying the carrier distance and time to launch aircraft.

In the example, Ark Royal and her destroyers continued ahead while Renown and a cruiser, probably Sheffield, fell back to cover them against pursuing Italian capital ships. The rearguard threatened damage and delay even if the stronger battleships could defeat it. Ark Royal would gain the sea room needed to launch aircraft, leaving the pursuers to anticipate air attack during a surface engagement. Uncertainty about British submarine positions added another reason to break off. The carrier's escorting capital ship could therefore buy time and distance for an air counterattack.