One can only guess to answer that question, as you don't tell us who has told you to do so.
One reason was already given:
If you or someone else has trouble with the fact that a non pressed button will result in a HIGH and a pressed button will result in a LOW, this is a hardware way to solve that.
Ofcourse you'd have to consider cost of the resistor (a few cents to buy it, some more to integrate it in your design and (relatively) a lot more space on your board) to that of programming in such way that you (so you, the Arduino doesn't give a rats *ss) will keep track of the status of that button / switch.