Well, It's very easy. Since computergames are becoming more realistic. It would be a good thing to use the render engine which runs on Directx or Opengl and which is hardware accelerated by the gpu to render out animated feature movies.
People always think of renderfarms as large computernetworks, but by adjusting the engine so with nvidia cuda it would allow you to do realistic renderings on a system with 4 sli enabled gpu (nvidia quadro cards). A game engine is programmable, so if you would like to do something like crowd simulation (massive marching soldiers like in Lord of the rings ) this could be much more easier done in a game engine (by using artificial intelligence scripting) then with a traditional render engine. You could also use level of detail meshes (high res meshes when the camera is close and low res when the camera is far away.) this renders much faster the animation. Scripting a render engine is really powerfull.
It would allow me to use a smaller computer setup to do realistic renderings for animations.
Some 3d content creating software like 3ds max is using Havok as their physics engine (havok is also used for computergames by the way. ) So if they need to calculate gravity they use Havok as their physics engine. Nvidia recently purchased Ageia Physics and are now integrating them into their gpu. So realtime physics can now be calculated by the gpu. This means you can calculate almost any effect with the gpu acceleration. THIS MEANS SMALLER CUDA QUADROFX BASED RENDERFARMS.
http://www.nvidia.com/object/nvidia_physx.htmlhttp://www.youtube.com/watch?v=ZiIfSCuwiQwhttp://www.youtube.com/watch?v=TU0fa8FUugg