Probably the same motivation as Hugh Jackman and Kate Winslet in the long scene in Movie 43 where Jackman has a set of testicles hanging from his throat: for the h*ll of it.
I doubt he did. You'd be amazed at what the current "deep fake" technology can do in the realm of speech synthesis and editing. There are also some very impressive voice impersonators out there. That said, it's still hilarious.Very funny, but what possessed Morgan Freeman of all people to narrate this?