How does a multinational manufacturer plan, in advance, the program for a supply chain that turns on a dime and may take into account a civil war, a tsunami, a stock market dive, a massive recall, a competitor's hot product introduction, and more? Well, no planning tool to date is this robust. It might be safe to say that no planning tool will ever be this robust. Does this mean you don't plan? Of course you do.
Planning is a guide; it is not what really happens on a day-to-day or hour-to-hour basis.
Source: Managing Automation, http://www.managingautomation.com