Question

Why do you start with the greater number when adding on a number line?

Answer 1

You start with the greater number when adding on a number line simply for convenience.

Put it this way:

Would you rather add more to a small number or add less to a large number?

Naturally people would choose to add less to a large number because that way you do not have to do so much work. So, you would start with the greater number when adding on a number line.