Why Britain armed auxiliary AA and ASW ships

Why did Britain create so many auxiliary anti-aircraft and anti-submarine ships?

They met extensive anti-submarine needs and supplied useful anti-aircraft protection for ports, rivers, and small coastal convoys.

Britain had many anti-submarine tasks for auxiliary vessels to perform. Auxiliary anti-aircraft ships could help protect ports and river approaches. They could also accompany small coastal convoys around Britain or in the Mediterranean. Armed trawlers supplied much of the auxiliary anti-submarine force.