In Windows 7 and afterward, when you open more than one Excel file, you still have only one Excel window and it is necessary to toggle back and forth between multiple files. Especially if you have two monitors, it is easier to have files open side-by-side.
To achieve this, you must modify a rather obscure Excel setting.
See http://superuser.com/questions/21602/open-excel-files-in-new-window
for step by step instructions.