How is this different from the existing command-line calculator 'bc'?

    echo "3+4" | bc 
    7

Other than not needing equations to be piped to it?

Note also that bc allows defining functions (among other features), and is also arbitrary precision (factorial of 200 below):

    $ echo "define f (x) { if (x <= 1) return (1); return (f(x-1) * x); }; f(200)"| bc
    78865786736479050355236321393218506229513597768717326329474253324435\
    94499634033429203042840119846239041772121389196388302576427902426371\
    05061926624952829931113462857270763317237396988943922445621451664240\
    25403329186413122742829485327752424240757390324032125740557956866022\
    60319041703240623517008587961789222227896237038973747200000000000000\
    00000000000000000000000000000000000