Simple question, as stated in the title.
What is the reasoning behind the arrows on the numpad of my keyboard?
They shift the cursor. One of the many residual features of a keyboard which was originally designed for the dumb terminals of a mainframe computer.
The 'number pad' on the right hand end of the keyboard is designed for heavy duty numerical inputting.
The 'number lock' button puts it in and out of number mode and as you can see in the original design 'number lock' is part of the keypad, so that when one handed 'data inputting' the user can flick between number and arrows modes on the keypad without moving their hand.
I actually found out that back in 1981, keyboards didn't have separate arrow keys, nor was there a mouse to use to move the cursor. So you needed the arrows on the number-pad to move the cursor around.
See this IBM keyboard from 1981 for example.
A long time ago it was thought that humans could understand modal operation of a keyboard.
Steve Jobs ideas of one button, and click to rule them all, soon created a trend amongst many designers in which it was thought modality was bad.
One knob, one button, per function, became the norm.
And making hardware became cheaper, and PC interface standards got loosened, too.