It's a relic of the past. Old keyboards didn't have dedicated arrow keys and since many programs were only controlled by arrow keys rather than a mouse, having a num lock key was a good solution to the problem. But if you are still stuck in an environment where you have to navigate a lot with arrow keys, the num lock still comes in handy. Having Pg Up and Down and the Home/End keys nearby is super handy in this situation.
It's still useful for some old video games, especially for multiplayer where both play at the same time.