The document body property of the document object can always be used to quickly gain a reference to the body element. So it can be used as an alternative to something like document.getElementById, or document.getElementsByTagName when it is only just the single body tag of an html document that is of concern. In this example I am creating a canvas element with the create element method, and then appending it to the body element of an html document with the append child method.
So for this example I might have some html that looks like this.
If all goes well this example just results in a black blank canvas, nothing to interesting, but you get the idea. So as you can see the document.body property is always there to serve as a quick reference to the body element.