What's the use case for knowing when you are looking at a fake AI image and not at a real photo? Is that seriously your question?

A few likely use cases,

1. Social media moderation. You want to limit the proliferation of this stuff because it annoys users.

2. Trades where authenticity matters, like journalism, law, education. You have to weed out the crap and people will try to deceive you.

eCommerce - you may want to reduce fraudulent listings, e.g. used goods, housing, collectibles/auctions...