Well, the worst thing that could happen if you don't have good cash flow is bankruptcy. If you're not able to pay your employees and suppliers, they'll stop working for you. This means that your business will be unable—or unwilling—to fulfill orders, and customers won't be getting what they need from you. Without enough cash flow, your business will fail.
Good cash flow is a crucial part of running any kind of business, but especially if it's a small one or an independent one. Small businesses are often dependent on the work of one or two people who may not always be able to find work elsewhere if things go south at their company; independent contractors and freelancers are even more vulnerable because their income comes from only one source. If that source dries up due to lack of cash flow, they have nothing else to fall back on.