Purchasing teams have a lot of different tasks they need to coordinate based on different spend management initiatives. Sometimes there might be categories of purchases that you want to control more and look for opportunities for special buys. With the purchase requisitions in AX you can use policies to control what you might want to do demand consolidation on. So each requisition would be accumulated so you might be able to have a greater quantity to make a spot by with a supplier an negotiate a discount. This video gives an overview of some of the setup and concepts in AX2012.