Manufacturing businesses rely on buying stocks from other businesses in the form of raw materials or components. Purchasing will ensure that the business has enough material so that it is able to operate. If the purchasing department has not done its job properly a factory will run out of stock and production will stop. This will mean that their customers will be left waiting for orders to be met.
Purchasing will try to negotiate the lowest price. This often means purchasing in large amounts, as they will receive a discount for buying in bulk.