Newbie Question: Why do you need a PreAmp?

Is it possible to bypass the preamp completely if I am not interested in DSP? I know the new Marantz DV9500 with the latest Cirrus Logic DAC has a built in volume control, and other souces like my PC, and outboard Dolby Digital Decoder have their own volume controls, so I guess my question is why do I still need a Preamp? Does it somehow add a voltage to the input source, or is it some other thing I am missing here?

Thanks,

Jason