It's been about 4 years since I've used Briar, so take this with a grain of salt, but I found it extremely useful at an event where I was normally within WiFi distance of my friends, but we didn't have cell service. Like, we might be in the same conference room, just not actually near each other, and the site had shoddy internet service.

If we could connect to each other's phones as "hotspots," we could send messages over Briar that wouldn't get through any other way.

It also worked well with a local mesh net set up around the conference location for file sharing, but that didn't have internet access.

Not the intended use case from what I understand, but it worked surprisingly well, and better than the goTennas we tried the year before!

From a Signal user: Sounds good! Never heard of it before, nor has anyone I know probably, no iOS app, not an option I'm afraid.