Imagine that you have two workbooks. One is the budget for the Sales department and the other is the budget for the Marketing department. You'd like to compare both workbooks to see the differences in projected expenses between the two departments.
As you can see in the picture, the Window menu shows that both workbooks are already open. Because Marketing is in view, the name of the second worksheet, Sales, is listed after the Side by Side command. To see both workbooks at the same time, you click Compare Side by Side with Sales.