Places like Red Robin and Applebee's offer AYCE fries with burgers, so I'm guessing McDonald's thought they'd try it too. Unfortunately, I don't think they realized that most people going to McDonald's don't want to linger after eating their burger to eat more fries.

I think Red Robin and Applebee's offer it as a way to increase the amount of alcohol they sell, because if you run out of beer, but still have fries, you'll likely order another drink.