I could say that it provides control for a proper ratio of fry to ketchup for every bite (but consistency and quality aren't the first words that come to mind around here).

So I'm going to go with it prolongs their eating experience. Stopping at a noodle or rice place, they will slurp it down as fast as possible. But to go out to McD's is a pricey adventure and they want that increase of face being seen there. So to individually squeeze ketchup on each fry doubles the time that people can see them eating there.

That being said, I guess I don't eat at places like that enough, because I've never paid any attention to how the Chinese eat their french fries (or been too intoxicated when I was there to care).

I'm not Chinese and I do that too. So it is not a Chinese thing, it's just a habit thing.

More fun?

Kill time？

There is a big difference between that and just dipping. The fries must get cold before they are done. Maybe they suffer from OCD?
I will have to pay attention next time I go to one of those grease peddlers.

If it's hot outside, they want to stop inside where it is airconditioned, if it's cold, vise versa, otherwise they are bored or probably worried about how fat they will get!!

Because in ancient Chinese culture Emperor Wang Zhi Ronald McDonald who invented the hamburger was inspired when he witnessed his great adversary Kubla Hamburglar fataly stabbed through the heart with a sliver of deep fried potato. The fry with the dripping of sauce symbolises the burglars blood that was spilled that day.

i also do the same,i am English so just a personal preference i think ,also stops you eating too fast