That’s the first time I hear about this, but apparently it’s a common practice in some Lebanese restaurants and houses to put water bags to keep flies away from their food.
I made a quick search and surprisingly it’s a common practice everywhere in the world. Opinions however are split on whether this really works or not. [Link]
As far as I am concerned, there were no flies to be seen in Al Erzel in Faraya where I was on Saturday, but it could be because of the weather.