Buffett tries to find stocks that are stable and understandable. By implementing this rule, Buffett can generally assess the value of a stock because he can somewhat predict their cash flow and earnings power. For example, if you were going to buy a company on Main Street, would you consider buying a company that made un-predictable profits, or would you find the company that generally produces similar results from month to month? Seems like a simple question when you look at it from that perspective. When a company makes similar profits from month to month, you can assess how much money it will make in the long-run, and properly assess a price or value. This is why Buffett tries to find businesses that are stable and predictable.