My Bible study class has grown enough that standard SMS/MMS group texts are hitting up against Verizon's carrier limit (I think we have only two people who use Verizon), and RCS isn't an option. Telegram wasn't my first choice, but it was a lot easier for everyone, and it supports tablets out-of-the-box.
1. You must join a group to see its contents. This puts a stupid "{user} joined the room" message in the room for everyone to see. Channel membership is anonymous.
2. Channels generate a public feed that doesn't require any special software to read from (see Durov: https://t.me/s/durov). Matrix requires a client of some kind, all clients are clunky at best and none have a public view.
3. Channels support millions of readers. Matrix rooms literally cannot do this, and rooms frequently get out of sync due to federation.