Bash functions enable faster workflows. Say you want to use the emacs calculator for a fast unit conversion. Instead of opening emacs and waiting for it to startup, then using C-x * *, you can write the following in your ~/.bashrc (or whatever your startup file is)

c(){
    emacs -Q -f calc
}

after logging in to bash again, entering c as a command will start the calculator. You take this a step further and execute a lisp expression

c(){
	emacs -Q --execute "(progn (calc)(delete-other-windows))"
}

To see just the calculator buffer and not the others.

emacs --help

Shows a summary of options when starting emacs. More examples:

emacs --batch -Q --eval (message (number-to-string (+ 8 9)))
emacs --batch -Q --eval (message (calc-eval "7+8")) 2>&1 | tail -1