[kazinator]

What buildn will do with the list is simply lose it. The last form can extract it and do do something with it.

When you might use it is when the goal isn't to build a list which escapes. The construct supports queuing semantics (insert at one end, take from the other), so you can use buildn to express a breadth-first traversal that doesn't return anything, or returns something other than the list:

  (defun bf-map (tree visit-fn)
    (buildn
      (add tree)
      (whilet ((item (del)))  ;; (del) from front
        (if (atom item)
          [visit-fn item]
          (each ((el item))
            (add el))))))     ;; (add) to back