I don't know explain, but this is the result of my tests.
Target: NGC 6496
Top photo - with Bias
Bottom photo - without Bias.
First photo: Canon T3
Second photo: Canon 350D - without internal IR filter.
Both photos have HDR convertion, adjustment in curves and level, in Phothop CS3.
Without Bias the image has less noise and the stars have better shape.
With Canon 350D I needed to reduce the intensity of red information. I haven't, yet, a good custom white balance to avoid the general redish color. So I have loose of stars color information, mainly blue. It was necessary to adjust it with reduction of Red in Selective Color-Red.