I moved house recently and this was among the stuff left along with extra fridge shelves etc (useful stuff, not rubbish!) but I can't work out what it is!
It seems to fit the oven shelf so it can hook over it and hang down but I don't know why it's angled and not square? Also if you hang it at the back of the oven it just 'swings' backwards so it's not level, so you can only really hang it on the front of a top oven shelf with another normal oven shelf below it to stop it swinging backwards. - so it sticks out of the door.
I'm sure I'm being completely dense and it's really obvious!