Well, I just add it because of what it may be doing, not what it I know it does.
Yeast needs more than sugar and water to ferment, it requires other nutrients of which some are found in water. Lots of time the reason your yeast stops is because of lack of the micronutrients, not because of alchohal levels being too much. I add flour because it has several nutrients which might be useful to the yeast. You are better off if you get "yeast" nutrient from your local brewing supply store, but I just add flour for what it could do.
Tank in transition! 55 gallons, hard water.