Unless you get up in the middle of the night for snacks, or you can eat while sleeping, you wake up having not consumed anything for (one hopes) several hours. You've been fasting. Breakfast ends that. It "breaks" it. And your body (one hopes) thanks you by getting more awake. You get more ready for the day! You may not be so ready had you not -- wait for it -- Broken the Fast!
In related news, my just-after-waking banana and apple juice taste really good.
