If fire controls don't have a specific target assigned, they'll automatically fire at missiles when they have a point defence mode set.
Of the PD modes, I think the final fire one works best, and I don't think the area defence mode really works. I guess the area defence one needs time for the fire control to get ready to fire, which in practice means it doesn't manage to fire at incoming missiles even once, unless the PD range is very long, or the missiles are very slow, but I've never encountered slow missiles.