It makes sense on a laptop. But, if I already need to have a clear place on my desk for the keyboard and mouse, I'd rather just use an extra 2 inches to have the full size keyboard with the numpad and arrow keys that are not crammed against the rest of the keyboard.
I don't use the number row above my keyboard except on rare occasions or to type the shifted characters. If I need to quickly type a number without looking, the numpad is the only way to do that (for me).
I tried a friends "compact" gaming keyboard, and then to the right of his keyboard was a separate "macro" keyboard which was basically just a numpad... so why not just have a numpad?
Numpads are specialist equipment now that data entry is much less common than the past, as everything is digitized/scanned.
The other advantage of a separate numpad is that you can position it for better ergonomics; a full sized keyboard usually forces either the mouse or the keyboard into a more awkward position.