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.
The asymmetric nature of having it at the right side drives me crazy.
I want my hands at the middle of the keyboard, not cramped to the left side, with so much space at the right, for keys I almost never use.
Having said that, my current keyboard has no numpad.
Not sure why this option isn't more popular tbh since you can put the numpad where you want it.
With the separate numpad you can position it just above the mousepad so it's still easily accessible, but you have minimal movement distance between typing and using the mouse. To me it's the best of both worlds, and it's strange to me that it isn't more common to sell the two as a package. I suppose the assumption is that people who want numpad will just buy a 90% keyboard and numpad separately.