I have a belay device that looks like:
which I used when I am belaying someone who is top roping or leading. However, my rope only goes through one of those slots. What is the purpose of the other one?
Most belay devices come with two holes for rope, for various reasons:
Your specific device (it is a symmetric design) would be best suited for scenarios 3 and 4.
It's a trick! Don't use it!
Rappelling/Abseiling is the answer though.
Some people use it for belaying twin or double ropes too.