Honestly though it looks like (from shape and dimensions) it has to do with the 3.5" bay. It looks like it holds 3 HDDs or something. Maybe closes off the 3.5" bay.
The HDDs in that case are meant to be mounted vertically in the internal drive cage. If you wanted to mount them horizontally you mount that plate in the cage vertically and then can attach HDDs using the side of the cage and that plate.