That's a fair point, but frankly, it was just an example of why one might want to use such a function to find arity of arbitrary callable. In this example I could also invoke callback with two arguments only if it wants _at least_ two. I.e.
(my-foo (lambda (x &optional y) ...)) ==> called back with one argument
(my-foo (lambda (x y) ...)) ==> called back with two arguments
Another usecase is possibility to fail early. I know that's not a popular concept in Elisp, but I find it very useful. The idea is, when you are given some value that you use only later, you validate it right away. Thus, you can give immediate error with explanation rather than letting it fail in mysterious and hard to understand way in a completely different place later:
(defun my-set-termination-callback (callback)
(unless (and (functionp callback) (>= (car (function-arity callback)) 1))
(setq my-termination-callback callback))