Lower video settings can mean a reduction in GPU load, video memory bandwidth required, CPU load, system RAM bandwidth required, amount of data to be retrieved from the hard drive...all this is quite deliberately transparent to the user.

Oh, and in some cases a multi-GPU setup will help WoW. Extreme resolutions at high graphics setings, for one. Especially, say, if you have several clients running which are each independently trying to talk to the GPU. Normally, no, because it's an old game and most of the work is because of the number of object to be rendered not the complexity of the rendering job. That last part is a little less true with Wrath, but it's still largely an MMO-type load being an MMO: lots of objects with limited individual complexity.