In the first part of its latest teardown, iFixit reveals just how tiny the logic board of the new M1 iMac actually is. Most of the interior of the desktop is taken up by two huge metal plates; all the computing hardware is contained on a board that seems to take up less than 10% of the interior volume.
https://www.notebookcheck.net/The-new-iMac-is-more-metal-plate-than-computer-according-to-iFixit-s-latest-teardown.540924.0.html
They make the chin so 2 metal plates can be on the back of the screen. Still ,if that 2 plates is a must they can put the board in the stand or base instead. So my conclusuon is the chin is there coz they want to make it.
...I was wondering what the plates were, and IIRC the two assumptions were heatsinks and speaker chambers, turns out they're...just plates. Unexpected, honestly.