Compared to a phone (SMS) and other messaging apps (whatsapp, email, signal), fb messenger has two big benefits for me:
* Cross platform: I don't have to type on a tiny screen when I'm in front of a computer, and won't lose my account or my messages if I lose my phone or phone number.
* Discoverability: most people who I meet are on it, so I can reach out to people I meet at events as well as friends of friends.
I don't love messenger. I use Messenger Lite on my phone to avoid most of the snapchat/gif/events/birthdays features. But I'm not aware of any other cross-platform messaging app with that discoverability, let alone one free of privacy issues.