In today's generation of ubiquitous calculating devices (smartphone/laptop etc), we hardly use standalone calculators, let alone take the trouble to mentally or manually do these calculation tasks.
One might argue that we've outsourced the ability to calculate, does that make us dumber?
Imagine having a teacher like Asimov, presenting concepts (in this case, arithmetic hacks and shortcuts) in a lucid and simple manner! The scope of this book is very limited, and hardly covers junior school arithmetic concepts, but is an interesting detour into the history of several of the calculations we take for granted.