Look at it this way... The data inside your computer is digital. The representation of windows on your desktop is digital. Your LCD monitor is a digital display, even though the final signal to the LCD cell is analog. Why convert your digital display representation to an analog Red, Green, and Blue form via a Digital-to-Analog (D2A) converter, send it across the cable to your monitor only to have it turned back into digital form through an Analog-to-Digital (A2D) sampler so your monitor can then display it?
DVI-D keeps it digital the whole way. Much easier and cleaner that way.