Explanation Request for the Function set-transient-map in subr.el

[xenodasein]

What does this section do? In what situations this-command gets called using 
keys of the transient map but it is not equal to itself? What does mc mean?

((eq t keep-pred)
  (let ((mc (lookup-key map (this-command-keys-vector))))
  ;; If the key is unbound `this-command` is
  ;; nil and so is `mc`.
    (and mc (eq this-command mc))))