Many instant messaging applications have this feature, you can see when people read your message. What is the purpose of this?
What came into my mind is to achieve a more natural way of communication. Making messaging similar to speaking; we can kind of understand when others hear us when we say something.
This function does exactly that what you consider more natural; let people know whether or not people got the message through less explicit communication. It saves a crapload of confirmation messages that are just "ok".
In real life, we can see if people are looking at us, how far away they are, if they're nodding or rolling their eyes. Basically, we get contextual feedback from which we decipher if someone heard what we said.
But when someone is half a country away typing on a black plastic slab, we don hear them, nor see them and all those contextual clues are gone. The tickmarks are an approximation of those clues.
The feature provides immediate feedback that the message has been read. This avoids having to send additional messages to confirm this. So the messages can focus on the actual content of the communication rather than the data transfer process.
