I mean, yes, Ctrl+Esc opens the Start menu, which the Windows key also opens.

But the Windows key -- before MS renamed it, the Super key -- is a single modifier key in its own right.

Win+R = Run

Win+D = hide all windows and show Desktop

Win+E = open Explorer

Win+SysRq = open System Properties.

There are dozens of them; these are just a few of the ones I personally use.

Other OSes and desktops use it for other things. In GNOME it opens the Overview. In Unity it opens the search function, and Super+A opens the app search function, and Super+S opens the virtual-desktop switcher, etc.

It has lots and lots of uses, and opening the Start menu is just one. Ctrl+Esc doesn't do any of these other functions, and it cannot do them because it's not a modifier key so you can't use it plus another key at the same time.

People often say that they can't use an old, pre-Win95 keyboard because they need the Windows key. What I am pointing out is that it's easy to work around that, by sacrificing another key that's not very useful: Caps Lock.

This also sits in the same location as Ctrl on the old Sun keyboards, which is a convenient position for a modifier key -- it's easy to press CapsLock with your little finger while using the rest of the hand to press the other key of the combination.