VSTO provides templates for creating Office projects like Excel WorkBooks and custom Ribbon Tabs.
You get to write .Net code to add behavior to the Excel workbook. It starts getting interesting to me when you add a web service reference to /MT/ERPBusinessObjectService/ERPBusinessObjectService.asmx.
With this combination you can create Manage 2000 awareness in Excel workbooks. This might take the form of item validation:
Or maybe Cross Referencing:
Or even reading and writing business objects: