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Because it's also used for shutdown and sleep keyboard shortcuts.

Option+Command+Eject will put your machine to sleep, for example.
 
As another poster stated, for the optional external super drive.

If not that then just to screw you.
 
Yea, its for some kb shortcuts like Ctrl-Shift-Eject (put the display to sleep) and also for ejecting disks if you connect an external drive.
 
Holding F12 will eject a disc as well.

Also - you can set up your smart corners (while holding control or shift) do put the display to sleep. Tis what I do with my iMac.
 
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