Typing and read receipts make a lot of sense for messaging. You write a letter, you put it in an envelope, you send it to a friend, and you want to know when they get it. It's like FedEx - they let you know when the package gets dropped off.
I think about the characters I've created, and then I sit down and start typing and see what they will do. There's a lot of subconscious thought that goes on. It amazes me to find out, a few chapters later, why I put someone in a certain place when I did.