In too many cases it is like the cashier who relies on the register to compute the required change, without it they are lost. Although apps can help at times there is no substitute for the basics and lots of experience.
One example would be designing a set of stairs. With one of the apps you don't know if it will work until you build it (expensive). Designing it by hand with a good sketch would greatly increase the confidence level.
As windows asked, what specifically are you looking for?
Bud