What about the nature of welfare during world war 2 made the ability to produce war material so important?

Because the war was wide spread and there was a need to supply millions of soldiers and sailors with the materials and machines they needed to fight. This was a true world war with fighting going on in much of the world.

As an example: the war in Europe might have lasted a year or two longer if it had not been for the ability of American to quickly produce all of the landing craft needed for the D-Day invasion.