Stuff can always be shipped from other parts of the world on the moment's notice if you have enough cash ready, so you don't necessarily need to stockpile it on site before anything happens. But what you can't really buy quickly is trained personnel that can organize relief works on site - this alone justifies having them as a standing organization.
I don't think that's really. Lots of speciality goods like survival shelters actually CAN'T be ordered in large quantities on a moments notice. Something like bottle water or MREs, sure. But not so much durable goods.
Most manufacturers these days run super-lean, often not stockpiling inventory at all.
