I think the numlock key is a hangover from the IBM XT computers (maybe even before that). Those keyboards didn't have the cursor keys and other key block. So, the numlock key was quite important.
I know, because I got my first PC during the PC AT days right before they moved toward ATX. That was a full size keyboard.
I am surprised it has stuck around so long. I understand in the transition from XT to AT that perhaps computer operators got used to using the numpad for navigation and muscle memory would be ruined. But, you know it's like 40 years on now. There isn't really a modern day reason to keep it.