Not all materials are parts. For instance, if you run a soup factory you don't order 'soup parts' but bulk ingredients in the same proportions that you end up selling, but there are many other things that you need to worry about in the process of preparation and there are things that don't even make it into the end product at all that you still need during the manufacturing step (for instance, cleaning products, various chemicals). So 'basically parts lists' is a bit of a shortcut.