"I may have invented it (control-alt-delete), but I think Bill (Gates) made it famous." — at a gathering at the twentieth anniversary of the IBM PC, whilst sitting in a panel with Mr.Gates([http://www.devilducky.com/media/28888/ See the video here.)
Bradley did not intend Control-Alt-Delete to be used by end users — it was meant to be used by people writing programs or documentation, so that they could reboot their computers without powering them down. This was useful since after a computer was powered down, it was necessary to wait a few seconds before powering it up again if one wanted to avoid damaging the hardware. Since software developers and technical writers would need to restart a computer many times, this key combination was a big time-saver. Bradley chose this key combination because it is practically impossible to accidentally press this combination of keys on a standard keyboard.
Control-Alt-Delete still exists in the Microsoft Windows operating system. It either gives a list of running programs or gives a list of administative functions, such as to reboot the computer, give a task list, or password protect the computer
The primary reason for using Control-Alt-Delete for these functions is because the kernel is the only process that can respond to it, while Control-Alt-Delete interrupts other programs. It is therefore a secure way to assure that the operating system is running, rather than a hostile program that emulates the operating system to record passwords or other sensitive data.