For a full size, 104-key PC keyboard, everything that is mapped to the numpad is also somewhere else, and the keys are spaced out enough that you'd almost never need to turn off the numpad, so the key is just there for oddball legacy apps that do weird stuff. It was more important before IBM released the fully "modern" 101-key Model M.
For laptop keyboards and other reduced format keyboards that still include all or part of a numpad, it can still be useful. I actually use autohotkeys instead, but one of my budget mechanical keyboards only has the arrow nav keys and the rest are accessed by turning off NumLock.