A built in, onscreen calculator would be useful on certain cases such as construction.
I see, got it.
I've built fences and gates using Canvas and a calculator. I needed one and found one that works with the phone doing fractions directly. Very handy, very useful. I also use an onscreen calculator when in Canvas but it does not stay in front. Just as a hobby, I am building a vacuum tube communications receiver which also needs calculations so my needs are different from yours.
I never found a need to have a calculator in Canvas, I use the calculator app that comes with the Mac. It has some nice advanced settings, including copy/paste results.