Are there any popular PWA-only apps without a mobile app counterpart?
File-sharing services (e.g., sharedrop?) seems to be some of them but I'm not sure if its popular in terms of usage.
It's somewhat popular for piracy, when you know that your site could never pass the Apple/Google store reviews. I've seen it with, for example, manga aggregators
There are a lot of PWAs packaged as mobile apps, probably more than you realize. Google makes this pretty seamless with TWAs, and you can use Capacitor/Cordova on iOS. You can even connect/write your own JavaScript interfaces to native APIs.
FWIW, in the US, PWAs are a non-starter for most apps due to Apple having so much marketshare and having horrendous UX for installing them. Anytime you have to provide your users "instructions" to do something on iOS, you've lost (and it's by design).