Have you ever read the terms? I don't know if they even publish them anymore, but I read one many years ago. TLDR;

1. The CA must misissue a cert.

2. The misissued cert is used by a malicious party to impersonate you.

3. Every user (your users) must prove their damages and claim individually.

4. There might have been a low maximum, per-user claim, but I can't remember.

I'd be amazed if there's a single person on the internet who's been paid out by that warranty.

Yes, it's useless insurance. The interesting thing is that useless insurance is illegal to sell in lots of places† - to consumers, but here the insurance was sold to the root CAs which are huge corporations so they don't care that it's useless because they only bought it as a PR exercise.

†This is the root of a huge scandal in the UK that resulted in banks refunding people years of fees for a product called PPI which they should never have been sold. As a secondary effect this resulted in annoying spam from firms who'd help you claim your money back. By the end I almost felt sad I hadn't fallen for the original scam, because I was being inconvenienced by all the spam but (since I hadn't lost anything) not getting a pile of cash as a refund.