The middle two seats are good for a couple who want to sleep on an overnight flight. Its dark and private in the sense that crew/pax walking along the aisle do not disturb you.
Its is not as good for day flights as it is harder to attract crew attention (but then that is what the call button is for) and you have to cross over the aisle pax if you want to walk around. You will also not really see out of the windows.
It is also good for people who are nervous about flying since the seats are the furtherest from the window and the view is usually blocked.