Roux whisks are perfect for blending a roux for sauces, such as bechamel. The flatness of the whisk means that it can mix right into the edges of the pan, meaning a smooth sauce every time.
They are also great for deglazing a pan or making gravy, as the widely spaced, flat wires lie flat on the pan and are able to scrape up every last bit of the goodness from the pan.