为什么add-hook允许`hook'无效？

来自C-h f add-hook:

HOOK should be a symbol, and FUNCTION may be any valid function.  If
HOOK is void, it is first set to nil.  If HOOK's value is a single
function, it is changed to a list of functions.

并从代码:

(defun add-hook (hook function &optional append local)
  ...
  (or (boundp hook) (set hook nil))
  (or (default-boundp hook) (set-default hook nil))
  ...

这有什么用？我认为它在某种程度上是有用的,否则它就不会存在......我只是想不出它的好用...