Because, with an alias it is more versatile, if you wanted to use is in a bot or as another script function you can. not all scripts are meant to be bot scripts. some are client side, which this one is..

If you want to use this with a bot just use something like..

on $*:text:/$[!@`]eta/Si:#:msg # $eta($2-)

syntax: [!@`]eta 1mb/s,10gb,100gb