It trains the eye to see hidden structures within chaotic expressions.
$$f(h) = h \sin \frac1h$$
$$f(x) = \begincases x \sin \frac1x, & x \neq 0 \ 0, & x = 0 \endcases$$ demidovich calculus
The book is a comprehensive collection of over 3,000 problems in calculus, covering topics such as: It trains the eye to see hidden structures