What are any inevitable use-cases of the unindexable array in Forth?

There's none. It's kind of like the "hello world" or "empty program" example for arrays.

What this page shows is that the CREATE ... DOES> construct is similar to a closure, except it can only capture one address (or reference in languages that don't want you to dirty your hands with addresses ;-) and it is always a global symbol (like almost always in Forth). It is still useful to encapsulate implementation details, though. Some people have said it is a basis for objects, which is a bit euphoric IMHO. You know, "closures are the poor man's objects, and objects are the poor man's closures".

An "unindexed array" is basically `char *` in C; the use cases are the same, I suppose.