most functional techniques wrap walking a loop into a few functions, like fold. writing loop constructs can be error prone. one is much less liable to make a mistake passing in an array into a function and have it processed using another function. idea being separation of logic. why care about loops when one only thinks in arrays and streams being processed? i don't think i've written a for loop in js in a couple years at this point with the help of libraries like ramda.
oh and loops constructs are not composable...