That’s true, aggregates can now be used for things other than relationships! That feature was added super recently, and is still having the tyres kicked (so to speak), and it’s a bit too late to update the book with this new info 
As for calculations, we do go into examples first on how they can be used for generally calculating things, like the years_ago that an album was released. We use them more in later chapters for things like the track number of an album, and policy checking who can manage album data!