For me, for example, he selling point is having an interface as slick as Whatsapp's, but not being tied to the phone, not having to necessarily give your phone number, sending whatever attachments you want, and getting a searchable log in the cloud.
Bots are also a popular feature with many users.
I - and I guess many with me - don't trust the encryption at all.
I assume the crypto might be broken, NSA and FSB and aliens might be listening but I don't care.
My reasons to use it:
Desktop support is way better than Whatsapp.
More possibilities (bots, channels mostly).
And because I don't want to support Whatsapp by being part of the network effect that keeps people on that platform.
The reasons I use private chats sometimes is just to use autodelete to protect my conversations with my wife against my kids.
[1]: https://wire.com
Secret chats on OSX has the same UX as secret chats on my Android client, fwiw.