Splitting a document into two separate views allows you to view and edit two widely-separated parts of a document at the same time, which means you can work on the first chapter of your novel in the top pane and the last chapter in the bottom pane, and jump between them simply by clicking the mouse—or by tapping F6 to cycle between the two panes, the ribbon, and the status bar. Word also allows you to change the zoom level in the two panes independently, as shown here, so that you can view multiple pages in one pane while viewing full-size text in the other.
You can split the window by dragging the splitter bar at the very top of the vertical scroll bar, or open the View tab on the Ribbon and select Split (or Remove Split to restore a single window).