Spice – sumac used for

It's actually spelled 'roux', and is a mixture of oil and flour, cooked to remove the starchy taste of the flour.

It's a great thickener any time that you don't need the sauce to be clear, and you have time to cook it down. I typically use it for cream sauce (including cheese sauces, such as for mac & cheese) and gravies.

As for benefits -- it's habit at this point, so I'm not really sure -- I guess it's made from things I always have on hand. You can also get flavor from the roux, if you cook it longer , but you'll adversely affect the thickening ability. Gumbo is normally made from a dark roux (the Cajuns have a series of names for the color of roux, including 'brick', 'peanut-butter' and 'chocolate')

One good use for it is to amp up the flavor of vegetarian gravy. If you are making say biscuits and gravy, a tablespoon of marmite will add some umami without tasting like soy sauce. Same for a vegetarian pot pie or stew.