Its also a way to get the nominee in place and not be held up in confirmation.
For example, POTUS wants to replace Sessions with Barr. To prevent Barr from being held up in confirmation, POTUS nominates Whitaker as ACTING AG. Whitaker goes in and starts declassifying and firing and causing so much chaos for the left that Barr sails through nomination just to get Whitaker out of there. Same thing happened with Richard Grenell as acting DNI when John Ratcliffe was the nominee.
IMHO