Operating margin picks up where gross margin leaves off. It is what remains after the everyday costs of actually running the business, such as sales and marketing, administration, and research and development. Because it captures both pricing power and how tightly the company is managed, it gives a fuller picture of profitability than gross margin alone.
Compare it two ways: against rivals in the same industry, where the gap between the leaders and the weakest performers is revealing, and against the same company's own good and bad years. A business with a strong moat holds its operating margin fairly steady even when sales dip.