The best way to ensure maximum throughput is to maintain a large buffer. A waiting room of patients is that buffer. No matter how many cancel or come late, the doctors/nurses will always have another patient "on deck". Zero downtime is the goal. An empty waiting room, and potentialy underworked staff, is waste.
But why mention it as the first item when specifically mentioning doctors - isn't most every kind of business or even non-profit ‘incentivized’ in that way - they can serve more customers if there is a large waiting buffer?
Choice and competition greatly alleviate that in other markets. With the medical system in the US, you often don't have much of a choice as switching doctors can be tough, and even if you can find one accepting patients, they are all controlled by the same administration and effectively collude to maximize their profits.
