The Apple logo is no longer just for looks, it now has a job to do. Earlier aluminum iMacs had an aluminum front and a plastic back. In contrast, this iMac features an all-aluminum enclosure, which is great for looks, but terrible for Wi-Fi. The Apple logo is the only plastic area on the back of the iMac, and Apple has integrated an AirPort antenna into that space.
Ah! so that’s what it is and if so I now understand the logic. Each time I have opened up my iMac I have pondered over what it was for. I have to confess although I have looked at the ifixit tear downs numerous times I had obviously not been paying close enough attention.