It isn't shown in TFA, but the zcalc keymap is bindable in the line editor so not only can you use it like qalc/irb/luap/whatever you can also take advantage of

It isn't shown in TFA, but the zcalc keymap is bindable in the line editor so not only can you use it like qalc/irb/luap/whatever you can also take advantage of any other functionality of zsh within it. Admittedly, you could do some of that with some magic $if/$endif guards in your ~/.inputrc if your REPL uses readline, but the integration is nice functionality if you're a heavy zsh user. I think of it more like the expression register in vim or eval-print-expression in emacs.