I always assumed that the first checkmark indicates "delivered" and the second one "read", since right after you post it it shows a clock-symbol.
also interesting, that i assume with current network speeds sent=delivered.
First checkmark confirms it hit store and forward servers. I prefer second checkmark to indicate delivery, as I care less if someone read it and more if it just made it to their device.
User you are sending to could be underground/out of signal range/have 3G disabled. I like having a separate delivered confirmation and I would prefer not to have people see what I have read.
Personally "It left the phone" state is very important to me, esp., when I am travelling and network connectivity is intermittent.
Also "Message Read" is a very ambiguous state according to me - If the content of a message is read from the notification drop down, but user never opened whatsapp to read it - is it read, or not ?
Definitely. You want confirmation it got to someplace same until delivered to the recipient. 1 check for got to server, 1 check for delivered to recipient device, and then "Read" underneath the furthest part of the conversation that has been read. Or I'm just crazy and niche.