Say that you want to report actuals from last year and budget from this year. Regular pivot tables can't do that. Starting in Excel 2010, OLAP pivot tables can do that. But, that doesn't help you if you have regular old Excel data instead of OLAP cubes.
However...take your regular Excel data through PowerPivot and it turns into an OLAP Cube! Thus, Named Sets become an option.
Chapter 12 makes me want to run every single data set through pivot table. I need to turn back to Chapter 3 to remind myself of why not to do that...