Malta's submarines and strike aircraft were already inflicting cumulative losses on Axis supply traffic. That attrition could still leave a trickle of supplies reaching North Africa. Force K's surface ships could attack an entire convoy. The aim was to destroy the convoy and stop the flow of supplies completely.
Chapter · 0:46-1:20
Why Force K supplemented Malta's submarines and aircraft
In 1941-42, Force K was a surface raiding force intended to stop Axis resupply convoys to North Africa outright. Malta's submarines and aircraft were already wearing that traffic down through attrition.