Peter opined:
"So such functions can also easy be rewritten as simple loops." (my emphasis)

Of course, there's always more than 1 way to skin a cat, but there are certainly cases where reentrancy is by far the easiest - and to rewrite it would be neither "easy" nor give just a "simple" loop.

Consider, for example, how you might implement the MS-DOS dir /s command without a reentrant function...
(Yes, I know you're unlikely to actually want to do that on an 8051, but it's a good, familiar example).