Many gravy recipes have you mix together the flour and fat and then cook it on the stove for a few minutes. It is said that this is to cook the flour and thus get rid of the floury taste. That is probably true, however I find that bring the eventual gravy to a simmer also cooks out the floury taste, so it’s an extra step that isn’t required. I think that cooking the roux sometimes makes it turn a nice brown color, which can make your gravy browner, which is nice. Even though that possible color can be nice though, I still don’t cook the roux and for good reason. Cooking the roux can actually make it less good as a thickener once you add the liquid. This is imprecise to me. I like to know that if I add 1.5 tablespoons of flour to 1.5 tablespoons of fat, and then add 1 cup of liquid, I will get 1 cup of gravy that is of good thickness. When I cook the roux first, it doesn’t thicken as well, which means I need to add less liquid. I find that skipping that step doesn’t adversely affect my gravy and makes it more predictable and consistent.