Mirror plates are the most cost effective way to fix a picture to the wall. Frames fixed with mirror plates cannot be dislodged by accident or disturbed by enthusiastic cleaners. They are also less tempting to opportunist theives, especially if they are screwed to the wall with security screws of some kind. Upside down mirror plates provide a hidden fixing, whilst slotted mirror plates allow for quick and easy removal of the frame from the wall.
Right angled mirror plates are a more specialist item, useful as a bracket for fixing an inner and an outer frame together.