The idea of also using it to pause scrolling console text, I think, originated in Linux.
That original keyboard did not have separate arrow and pgup/pgdown (cursor control) keys, and numlock was how one toggled between the calculator keypad being 'numbers' vs. cursor controls (see the image above). The current 'separated' cursor controls and number pad layout arrived sometime during the IBM AT era, and at that point the 'numlock' key started to make less sense. It was kept around for backwards compatibility with old software that used the state of the numlock key to change its behavior (and/or that relied on the exact scan codes output by the number keypad in combination with the state of the numlock bit).
Then, one day, on a bare terminal, output was flying by that I needed to read. The lightbulb went on, and I pressed the key.