Why do some computer keyboards have a "NUM LOCK" Key?

You can download macros which let you reassign keys. Numlock was for incomplete keyboards without a number pad.

When you have to type a whole string of numbers, it can be faster than using the alternate number keys. Computer manufacturers are always reluctant to decrease functionality.

There are certain programs where you want to use the numpad as arrow keys while others you want the numbers, and the “NUM LOC” key allows for that.

Personally, I turn the NUM LOCK off when I am not typing for just that reason; easier access to the arrow keys, Page Up/Down, and Home/End.

As for “incomplete keyboards”, neither of my laptops have a NUM LOCK key; there is a different keystroke (Fn + F11) that does the same thing.

NUM LOCK does not just convert number keys into arrow keys. It toggles it back and forth. Therefore:

If NUM LOCK light is on, the keys are LOCKed to NUMerals.
So hitting 8 outputs “8”

If NUM LOCK light is off, the keys are not locked to numerals.
So hitting 8 outputs “up arrow”