It's mostly about keyboard navigation.
When the window has focus the tab order may begin at the address bar or in a sidebar of the browser.
If you open the page with no anchor, calling document.body.focus() onload will start the tab order on the first focusable element on the page itself.
You can call document.body.focus() to remove the focus from another element that has the focus.
This has the effect of passing subsequent key events directly to the body. It also resets tab key navigation, sending the user back to the beginning of the page, which can be a problem. It is often best to focus on some element of the body, rather than the body itself.