The point of low cost weapons is to give you options on high intensity warfare: namely your high cost weapons take out air defense capability, so you can stop using them and use cheaper more numerous systems to hit the now undefended targets.

The other benefit is just complicating air defense: put a lot of incoming in the air that can't be ignored, and makes it harder to find the higher spec systems mixed in - e.g. stealth systems when there's a lot of unstealthy platforms or munitions also attacking are going to be much harder to find.