Even though you shoot manual, it's still handy to play around with the exposure compensation feature once in a while. I shoot in manual mode most of the time, but in order to get my exposure right I might switch over to Av or Tv to get me close to the right exposure so I know where to start in Manual. If I know I'm going to want to underexpose the scene, I can find out what settings the camera believes will underexpose the shot, and go from there. It just cuts down on the time it takes to dial in the exposure while in Manual mode.