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.