The Mac Pro itself is a server-grade machine running Mac OS when you look at it. A lot of people, mainly developers, scientists, etc use Mac Pros since otherwise they'd be bottle necking themselves on possibly slower hardware. The Mac Pro I also believe is one of Apple's only machines that will come with the Intel Xeon "Nethlahem" chip or for that matter with any Quad Core CPU at the moment, the rest of the cheaper Macs such as the Mac Mini, iMac, and the Macintosh laptops all come with really a Core2Duo CPU.
Truthfully, 99.9% of the people out there would have absolutely no use for 128GB of RAM except for those who are really doing some heavy lifting computing/designing or who want bragging rights, or the ability to set up massive RAM drives for loading speeds that will pretty much max out any processor while content loads.