Until Excel 2013, when you start Excel a new workbook is automatically created and, if you have a workbook called Book.xltx saved in your xlstart folder (C:\Program Files\Microsoft Office 15\root\office15\XLSTART or C:\Users\USERNAME\AppData\Roaming\Microsoft\Excel\XLSTART), the new workbook will use Book.xltx as its template.
Excel 2013 doesn’t start with a new workbook by default but instead displays a welcome screen. The Blank workbook on this welcome screen is not based on Book.xltx. The New command button and the Control+n keyboard shortcut do still both use Book.xltx.
You can ensure Excel goes straight into a new workbook based on Book.xltx rather than displaying the welcome screen by turning off the option in the Start up Options section of General Excel Options: ‘Show the Start screen when this application starts’.