The original idea was simply to reset early PCs without turning them off. Microsoft adopted control-alt-delete to help ensure people powered down correctly, then to handle “administrative functions” such as the vital “end task” feature for computer software that crashes or otherwise gets stuck.
Bradley chose the control and alt keys because he needed two shift keys to make the operation work, and he chose the delete key because it was on the opposite side of the keyboard. He didn’t want people to hit control-alt-delete by accident.
I like how Bradley also says that “then it turned out that this reset was a problem-solver for people who were writing the programs and writing the instruction manuals.” They’re always blaming the technical writers.