I imagine Stephen Hawking would have found a use for this had it been available before everyone got used to his computer speaking voice. Anything that may cause someone to lose their ability to speak along the lines of your example.
Another might be for placeholdering - you could use an array of (licensed and used appropriately) voices to pitch a TV show, film, radio show, podcast, etc to give people a decent idea of how it would sound to get financing and hire actual people to make the real version. Ofc you'll need an answer to "why don't we just use these AI voices in the actual production?" from people trying to save a few quid.
Simple one- for fun. I'm considering AI cloning my voice and tinkering around until I find something useful to do with it. Maybe in my will I'll open source my vocal likeness as long as it's only to be used commercially as the voice of a spaceship's main computer or something. I'll be a sentence or two factoid in some Wikipedia article 300 years from now, haha.
Universal translator - if an AI can replicate my voice it could have me speak all sorts of languages in real-time.. sucks to be a human translator admittedly in this use case. Once the tech is fully ironed out and reliable we could potentially even get rid of "official" languages (eg you have to speak fluent English to be an airline pilot - heck of a learning curve on top of learning to be a pilot if you're from a country that doesn't teach English by default!)
I dunno if it'd be a weird uncanny valley thing, I wonder how an audiobook would sound reading a book in my own voice - unless I'm fully immersed in fiction that's generally how I take in a book, subvocalising with my voice in my head - maybe it'd help things bed in a bit better if it's my own voice reading it to me? If so I wonder how fast I could have AI-me read and still be able to take in the content with decent recall.. Might have to test this one!
Splintering off the audiobook idea - I wonder if you could help people untrain issues with their speaking in this manner? Like would hearing a non-stuttering version of their voice help someone with a stutter? I am purely in the land of hypothesis at this stage, but might be worth trying! Even if it doesn't help in that way, the person with a stutter would at least have a fallback voice if they're having a bad day of it :)
E: ooh, having an AI voice and pitch shifting it may help in training your voice to sound different, as you'd have something to aim for - "I knew I could do it because I heard it being done" sort of theory. The first example that popped into my head was someone transitioning between genders and wanting to adjust their voice to match the change.
I imagine there's other fields where this may be useful too - like if you wanted a BBC news job and need to soften out your accent (if they still require Received Pronunciation, idk)
Admittedly I could probably come up with more abuse cases than use cases if I put my mind to it, but wanted to stick to the assignment :)