Exposure values (eVs) translate f-stop numbers into a linear scale, which makes it much easier to compare them. It's very helpful when using a spot meter to do a scene brightness evalutation. Say, for example, that you take a reading of the grass in a scene, and it reads eV 2 .... and then you take a reading of the sky and it says ev 9, then you know that the sky is 7 stops brighter. It just makes the mental calculations alot easier.