It's also a great place to tack post-it notes.
There's a reason ergotron puts handles on many of its monitor mounts.
The iMac is basically the same as the M4 iPad Pro, and the iPad Pro doesn't have a chin.
Cooling seems like it might be a factor here. The iMac's display is probably going to be run at a brighter (and thus hotter) setting AND it's more likely to be used to do things that require high load for extended periods of time, so putting it in its own space probably helps.
They should just have separated everything in the foot, that would have made sense. Some sort of modern Sunflower iMac if you will. But Apple is more obsessed with thinness than practical design, so we get an impossibly thin iMac will all the flaws that brings...
Those all do have to go somewhere.
Now you could argue if it needs to be that thin but for the current configuration, there's nothing you can cram behind the screen.
They can't put the compute in the back of the display itself, while maintaining the same thickness like an iPad (which has the same CPU), because the room behind the displays is dominated by the speaker system, allowing the iMac to have surprisingly good audio quality despite being so thin.
[1] https://www.ifixit.com/Teardown/iMac+M1+24-Inch+Teardown/142...
I miss the times when they used the form factor to actually make new shapes - both the sunflower and the cube looks more futuristic than the 2024 iMac.