You're right that the e2ee part is only about protecting the data while in transit, but that is because it's the hardest part. Apps can also store the data in an encrypted format so that other apps won't be able to read it.
I find it most useful as a means of getting answers for stuff that have poor documentation. A couple weeks ago chatgpt gave me an answer whose keyword had no matches on Google at all. No idea where it took that from (probably some private codebase), but it worked.
Squaresoft games were so good that there was a weird full decade there where the name Square Enix still managed to get me interested into checking out games, but the games themselves never did. Eventually this too died out and I finally don't care at all about square anymore.
It's an old joke about mixing up the words Polyglot and Troglodyte