Amazon (AMZN 0.57%) is best known for its e-commerce empire and its highly profitable cloud arm, Amazon Web Services (AWS). Further, ad revenue accelerated again in Q2, and recent management commentary points to more opportunity ahead. Although management doesn't provide specific commentary on its advertising margins, it often implies that they are key to the company's profit growth story. But advertising strengthens the case that Amazon can sustain double-digit top-line growth and rising profitability alongside AWS over time. AWS may be the more important profit engine today, but advertising is a quiet force that is enhancing Amazon's profit engine.