...this way you know if the person you sent the message to has read it or not.
Actually, you can
see, whether someone has been reading a PM sent by you!
As long as your PM resides in your Outbox
, the recipient has not yet read it. You could even still edit your PM in that state.
Once the PM is in the Sentbox
, the recipient has read your message.
Hope this helps,
[EDIT] Oops, seems Graham was one minute faster in typing the answer