Using the Extract tool only mimics what you can accomplish using other masking tools. However, the interface allows some interesting advantages, mostly for making the creation of masks more simplified and a bit less painful. The Extract function is not a substitute for complex manual masking, but under the right conditions it can create effective, complicated masks quickly. You probably won't want to use it when you have to make a simple selection or mask, and you also won't want to use it for very complicated masking. Complicated masking sometimes requires playing one mask against another, and not only does the extract function not offer this advantage, but the result is an applied mask rather than the mask itself. Without the mask, the extraction may not always be useful.