Anyone can screenshot anything that is visible. Encryption doesn't protect against that. But

1. People cannot verify screenshots. They can't decrypt by themselves.

2. It also protects who has permission to write in the group. Imagine syncing a presidential campaign via chat where only the top aides can write.

3. MLS reduces the cost of distribution of messages from having to encrypt to each receiver to just going down a tree of keys. The benefits is on reducing that traffic.