It's been quite awhile since I was primarily doing 3D animation work, but in the early days SG was using Unix and the software for 3D animation, required the horse power to do much of the work in real time for the previews. Back then everything had to be rendered on the PC to see what you did and how it looked. ( I worked with Topaz and 3D Studio, beta tester for Yost Group ver 2, 3 and 4) I don't know how these SG machines fare with todays processors but I imagine they continue to be the machine to handle such stuff like real time life like motion replacement technology. Instead of what we were using called inverse kinematics and scripts, where you had to do your own timing and bending of the wireframe for stuff like walking, running etc. It all looked fake. the SG platforms were using real people loaded with sensors to act out a choreograph and then this was, in real time, moved to the wireframed characters in the program with included texture maps for realtime viewing of the action.
A good show to attend to see what the differences are between these high end SG platforms and their high end software apps is SIGGRAPH. I haven't been to a show in years but that used to be the show for seeing all the high end stuff against the smaller PC stuff and capabilities of each.