This is a small but useful part if you have an IKEA Kallax standing freely as a divider. If you have any of the various boxes, this keeps them from dropping out from the back. Additionally, you could fix these (using screws or rivets) to the extra holes on the sides.
There's a full version for two boxes on each side and also a half version that can be used if you only need a stop on one side.
(And yes, I realise that something is really off with my print settings right now. Could be the rounding on the bottom, but the first layer looks like sh*t right now regardless.)