Lens Hoods cause two thing:
1) It will reduce spurious light from entering your lens, reducing flare
2) It can reduce the reflected light from the object your trying to photograph
Sounds goofy, but depending on the light surrounding the object that you are taking a picture of, your in camera light metering system may be defeating you.
Try this out and see what happens:
1) Set Camera to Matrix Metering (or whatever your camera calls full frame metering), take a photograph in a bright sky with a plane in it, both with and without the lens hood.
2) Set Camera to Spot Metering, take a photograph in a bright sky with a plane in it, both with and without the lens hood.
My D300 works completely different in each case......