Why do text files end in Ctrl+Z?

Actually, text files don't need to end in Ctrl+Z, but the convention persists in certain circles. (Though, fortunately, those circles are awfully small nowadays.) This story requires us to go back to CP/M, the operating system that MS-DOS envisioned itself as a successor to. (Since the 8086 envisioned itself as the successor to the 8080, it was na...