Hi,
I can see in Badoo.com a very cool and addictive function.
It is simple : The purpose is to be able to know (with a green tiny check icon) that our last message has been read or not by the person who is chatting with us.
The same way we already know in arrowchat when they are typing an answer thanks to the small chat icon which is appearing at the bottom right of the chat window.
Have a good day