It is the first Monday of the month in Metro Detroit and you are matching a stack of invoices from four different suppliers. The item you bought in March costs more now, a delivery charge showed up that nobody agreed to, and two invoices do not match anything you remember ordering.
With purchasing set up right, every order goes out on a purchase order with an agreed price. You have a short list of preferred suppliers, a written price agreement with the main one, and someone who checks each invoice against its order before it gets paid.
The surprises stop. You know what you pay for your top items, you collect the rebates you were already earning, and when a supplier raises prices you hear about it before your margin does.