As a photographer, its important for me that I shoot in manual mode on my camera. It all stems from using film in the past because you needed to know your settings in order for your film to turn out. (haha sorry for the run on sentence!)
Shooting in manual mode, you have complete control.
When I adjust the lens aperture and the shutter speed, I can achieve the desired exposure. Sometimes its fun to play, but for the most part I like to shoot as if the image is ready for only a few slight adjustments in photoshop. Many photographers choose to control aperture and shutter independently because opening up the aperture increases exposure and a slower shutter increases exposure but also increases the opportunity for that darn motion blur (but lets be honest, sometimes a little blur isn't all that bad). It all depends on what you're shooting. Here's an example of the 1st image that was shot at 1/100 @ F5.6. The image is basically over exposed, washed out and way to bright... however it's not a waste and don't delete it! Later, I took the image in photoshop and dropped the contrast down to give it that deep contrasty look (shown on photo #2). Viola.
The third image was shot at 1/100 @F13. A perfect example of how you can play with exposures.