There's not a lot of venting on the back (nor ability to install a fan in push mode), so it's likely as a means of moving additional heat out of the PCIe zone, and pull it out through the PSU (not as hot when mixed with cool air drawn in around from the front of the case past the ODD's, so it shouldn't be hot enough to cause damage to the PSU).
That all makes sense, but again, the plate is without vents. And even if they would go through the whole plate (which they don't), the PSU itself is still sealed, so no way to get hot air from the back of the PCIe section. If it gets any air from that section through the small holes that are in the plate that separate the sections, it gets cold air from the PCIe fan since the PSU's fan sits in front of the unit in close proximity to the PCIe fan. (wow, even I don't understand what I just wrote... ).
Edit: This is a shot from my 2009 machine. As you can see, the plate doesn't have vents. This is the ODD bay but the part where the PSU sits looks the same.