The main way I specialize messages at this point is basically 'Am I going to want this later'? If the answer is yes, I use email. If not I use Signal. It's interesting this was the most requested feature... it wouldn't be for me even though I love Signal.
It's important for Signal groups, because on a new device without a backup the groups you were in don't show up until someone sends a message in the group. Say if you were the only admin in an announcement-only group, no one else can send a message in the group, so that group is now lost to you.