Purchase Orders set the standard for receiving and invoicing to follow. They establish the accounting to be used, the shipping requirements, the timing and the quantities. Contracted pricing is clearly committed to as well
Orders become a mini-contract with suppliers, acknowledged through workflow by both buyers and suppliers and committing to the budget on the accounting side
Orders need to be changed, but the implications can be minor (a push out of the date) or major (price and quantity changes). For the latter case, a provisional requisition can be used to get approval of the change and then enact the change officially.