It's quite a long process to get needed items ordered, whether it's a big or small order. Since the process takes a while, individuals and departments need to plan ahead what they will be needing.
Getting the purchase requisition approved on various levels may take a while too, because those kind of things tend to sit on manager's desks for a time.
In the past, a lot of companies used to keep some petty cash handy for small purchases that didn't need a formal requisition. I wonder if companies have a petty cash fund anymore.