Some may have a shelf-life. They must be received, handled and stored - all adding to cost.
Manufacturing transforms raw materials into a finished product.
Operations grind to a halt without raw materials supply buying in quantity and having it available offers convenience and cost benefit - better than operating from hand to mouth. Quality acceptance tests may be needed on raw materials Finished product Finished product is stocked to be ready for customer sales. It also buffers manufacturing from preditable or unpredictable market demand. The firm can then supply from a well-managed capacity plan. Making a large batch may be cost effective even when demand is low.