It’s interesting reading everyone’s perspectives on what the Histogram is for as well as their thoughts on automatic exposure. This is my spin on it:
The Histogram tells me the dynamic range data of my shot and indicates whether I have my highlights or shadows properly exposed. By observing whether it is slanted to the left or the right, I can tell whether my exposure has been weighted towards the highlights or the shadows, and I can then adjust it either by switching over to manual or using the exposure compensation dial to fine tune the image.
The histogram is very useful for me especially when I am shooting on location and I can’t view the preview screen properly. To be honest I rarely rely on the accuracy of the preview screen and always use the histogram as a checking device to ensure my image capture is exposed properly. There is no correct or incorrect histogram to look for, it just depends on what you are trying to achieve. Given I always shoot in RAW, I have quite a bit of flexibility in the image in post production.
As far as automatic exposure goes, it certainly has its place. 60% of the time I shoot in Aperture Priority. The other percentage of time I bounce between manual and Shutter Priority. It all depends on what you are trying to achieve in your shot. I find by setting it to either Shutter or Aperture priority and using the exposure compensation dial to fine tune the exposure, I can obtain the image I am aiming for without going into manual mode.