What does flushing mussels in water with flour do? On Serious Eats, I read the following comment under a recipe:


  Why not flush before cooking? The correct spelling and translation, meuniere, means mussels a le miller’s wife. Because they add a bit of flour to the flush water before cooking. 


For clarity, this is the comment I refer to:


If I understand correctly, the comment refers to soaking mussels in water with flour before cooking them. The water is often used to get the mussels to release some unwanted things in their shells, but I'm not sure what the flour does.

After searching a bit, found that Meunière is a French technique for cooking fish. It involves drenching the fish in flour to fry them afterward. That makes sense as the flour, some fat, and the high temperature make some sort of crust or roux. 

When cooking mussels, however, there's liquid during the cooking to prevent reaching that high temperature. I've found that a roux cooks at temperatures upward of 300°F (~150°C), which the steam will not reach.

So my question is, what does the flour do when adding them to the mussels' soaking water? Is there any benefit, perhaps to the resulting stock (the linked recipe uses the stock for the sauce, but doesn't use flour)?
I think you are confusing Meunière (mostly for pan fried fish) and Marinière which is one way to prepare mussels.

Cornmeal is often used to clean mussels; it is supposed to help remove grits and dirt from the mussels.