Bathing is a truly great behavioral enhancer. Wetting the featheres encourages the Quaker to preen the feathers, which is a time consuming, somewhat tiring, endeavor. The more time your LGC spends completing constructive, self rewarding behaviors, like bathing, the less time he has for screaming, plucking, and biting. Bird feathers are covered in oils secreted from the birds skin. When feathers become too dusty from dander and dirt, the oil no longer spreads throughout the feathers, and they are neither protected, nor conditioned.
Dirty feathers harbor bacteria, become matted, itchy, and eventually cause illness and misery.
Although diet is the major contributing factor to feather condition, hygiene is obviously also important. My Quaker, Tango, loves to bathe in the bathroom basin. He despises the stainless steel watering bowl and kitchen sink, so they are not an option. I think this is related to the fact that his toenails make a 'nail on chalkboard' sound effect against the metal. Sometimes placing a paper towel or washcloth in the basin makes the bird more secure.