Origin of the forced restart command "Ctrl + Alt + Del"

It seems that it was invented by David Bradley, an engineer who worked at IBM.

For what reason did the forced restart command be pressed simultaneously with Ctrl key, Alt key and Del key?
Ctrl-Alt-Del inventor makes final reboot | The Register

David Bradley (engineer) - Wikipedia, the free encyclopedia

Control-Alt-Delete - Wikipedia, the free encyclopedia

Until then, when I thought about restarting, I was pushing the button to reset the machine itself or switching off the switch, but without such troublesome operation, restart with just the operation of the keyboard in front of my eyes It seems that it was set by saying that it is convenient if you can.

At this time, I chose the "Del" key which is located furthest from "Ctrl" and "Alt" and usually does not combine with these keys.

After that, Microsoft Bill Gates adopted this key combination, so it became remarkably famous.

The movie the person himself talks about the matter that selected this key is here.
http://www.devilducky.com/media/28888/

Bill Gates & amp; the ctrl + alt + delete shortcut - YouTube