Secondary info could have several interesting data options. First that comes to mind, is brightness for lights, but also levels for covers and/or volume. It would be great and flexible if we could specify the attribute to display. It would be even cooler if we could use a state or attribute form a different entity. That could be very interesting in combination with template sensors. I’m no programmer so i’m not quite sure how much i’m asking here, but just the brightness would be awesome imo.
EDIT: Another good use that comes to mind is previous state. That could be very nifty for some stock market sensors.