The only issue I have with anything said so far, is that color gamut is not related to the number of bits, or vice versa. Color gamut is the range of color from the lightest to darkest, usually broken down by the primary colors. Bit depth is a measure of the fineness of the gradations within the color gamut.
Where this matters, as Wizzard mentions, is when trying to recover details in very dark or light areas. Changing the luminance curve in these areas creates larger steps between colors, and more evident gradations. Having finer steps to begin with can make all the difference.