Sometimes circumstances beyond a person's or an organizations's control arise so you need a buffer period if work falls behind. Recently, in northeast Ohio we have had some severe snow storms. No human has control over mother nature. My place of employment has had to cancel work five times already this year (2018) because of weather. It is simply unsafe to travel on ice covered roads with negative 0 wind chills. We got behind in our work at no fault of our own, you can only do so much work from home. The slack is sometimes needed to get quality work. Most professors, clients, and customers would rather have a quality product a couple days late than a product that is rushed and low quality.