I am working on a project right now where I am rendering atmospheric effects (lots of clouds). Even on a 12-core workstation, it takes a very long time to render a single frame to even be able to get a good idea as to what things will look like, so I'm doing a lot of changing something, waiting… changing something, waiting.
Most of the final rendering can be offloaded to a farm, but being able to quickly and interactively get a good idea of what something will look like is extremely helpful and time-saving. Right now, I'm doing some of the work on an EC2 instance with 48 cores (96 vCPU) to help with this—cost-effective for occasional use, but I can certainly see the appeal of a machine like this. And for task like compositing where you need access to a bunch of huge source files, or where maybe you have a specialized video I/O device you need to use, running in the cloud might not be practical at all.
Not for the renderer I am using (Terragen), which has specialized features for rendering skies. I do normally use GPU rendering when it is an option though.
