Popularly associated with Microsoft Windows, it actually finds its roots in the IBM PC which was developed in early 1980s when Microsoft Windows did not even exist.
Back in the old days (70s & 80s) developers had to reboot their computers frequently to test their boot software and the only way to reboot was to switch off the computer and again switch it on. Since the power supply system for computers those days would take some time to switch off completely (due to the time required to discharge large capacitors), developers were advised to wait for few minutes after a switchoff before switching it on again to avoid damage. Due to this, IBM PC developers had to waste lot of time in waiting which could otherwise have been used for productive work.
To solve this problem, an engineer named Bradley in the IBM PC team designed an interrupt which would be triggered upon pressing the combination Ctrl+Alt+Del keys. This weird key combination was to ensure that even when something drops on the keyboard, it must not be unintentionally triggered. It required the developer to use both his hands with an intention to reboot and hence that combination was chosen.
This trigger was handled to soft-reboot the computer without the need to interfere with the power supply. Developers were happy because it saved their time and there was no need to physically get up from the chair and restart, they could do it from their keyboard itself.
Bradley had done this only for the benefit of his IBM PC team and never intended to expose it to the user. However, when Bill Gates later developed MS-DOS & Windows for the IBM PC, he ensured that such a “feature” is available to the user as well. One of the main reasons was because DOS & Windows were notorious for getting stuck abruptly (system hang) during which the user had to reboot the PC (Already frustrated over the system hang, users would have become much more furious if they were supposed to get up, reach for the switch, turn it off, wait for some time and then turn it on). However, there used to be some rare instances when the system would not respond even to Ctrl-Alt-Del (called hard hang) and the only way was to use the physical power button.
As the power supply systems for computers evolved in the 90s, their responses because instantaneous and reset buttons were included in the PC because the system could handle immediate off-on. Due to this, the Ctrl+Alt+Del started to become redundant and that was when it was decided to use it instead as a way to invoke task manager or lock the PC.
Although Bradley from IBM designed it, every PC user associates it with Bill Gates. Even during a recent interview, Bradley tried to pull Bill’s legs by stating: “I may have invented the Ctrl+Alt+Del, but I think Bill made it famous”:
In another recent interview, Bill Gates admitted that it was a mistake and blamed IBM for it: