To summarize:

Even though we’re all used to pointing and clicking these days, plenty of keyboard shortcuts are still incredibly useful. But did you ever take a moment to think about where some of these seeming random button combos actually came from?

The engineers behind the original IBM PC in the early 1980s, wanting a quick way to restart a computer using the keyboard, but they didn’t want to make it too easy, less the user accidentally reboot the computer and lose whatever they were working on. First, control escape was proposed, but all three of those keys are on the same side of the keyboard and they could be pressed easily by mistake.

Then Delete was assigned to be the third key. The key combination was originally intended just for development use by computer engineers, but it was leaked to the public in some IBM reference documents.

The combination has actually done different things in each version of Windows, but nowadays windows 10 gives you a screen with several useful options when pressing control alt delete.