Essentially, it turns an Android phone into a very sophisticated alarm system with real time notifications and streaming. So you can e.g. protect a hotel room using an old burner Android phone.
I use it against some creepy UK landlords and agents, who have the strange habit of breaking into rented flats when I'm not in, and without any prior notice. Totally illegal, but very hard to collect evidence against. Furthermore, contracts usually contain (illegal) clauses preventing you from installing a real home-alarm system. (Edit: contracts usually also ban changing locks).
It's also much better than any cheap alarm system, and totally complementary too. If something starts beeping (e.g a fire alarm or a motion sensor), Haven will notice and message you.
[1] https://f-droid.org/wiki/index.php?title=org.havenapp.main&r...