I've seen this sort of thing a few times:
(mapconcat 'identity '("" "home" "alex " "elisp" "erc") "/") => "/home/alex /elisp/erc"
mapconcatwants a function in the second position, hence just shove it the "identity" function if you don't want anything done to your input list first. But this
(defun shk-yas/helm-prompt (prompt choices &optional display-fn) "Use helm to select a snippet. Put this into `yas/prompt-functions.'" (interactive) (setq display-fn (or display-fn 'identity))
has me confused. Likewise this
(org-element-map (org-element-parse-buffer) 'headline #'identity)
I assume it has something to do with the code calling a function, but we don't want to give it one -- other than something reflexive like an identity. Again, the first example is clear, but not so much the next two.