The Cell is good at performing any kind of CPU functionality, as long as it's the code is well structured for the Cell. Written in very clean ways. Basically anything that runs on the PPE (PowerPC) can run on the Cell's SPUs if properly rewritten. The US Air Force Research Laboratory gives some insight why the are using PS3s:
"Before it won the research award in 2008, the information directorate's advanced computing architectures team considered alternative configurations and the possibility of a hybrid system, but found multicore Xeon servers slower and more expensive than PS3s, and GPGPUs to be slower in some important types of calculations. "
It was clear that too much redesign would need to be performed with regard to legacy game engines to make optimal use of the Cell's SPUs to fully take over all the dedicated tasks of a GPU. So that's why Sony in 2002 partnered with NVidia to design the RSX GPU for the PS3, instead of using a multi-Cell design. That's a year before Microsoft partnered with ATi for the 360 GPU, despite that console releasing a year earlier than the PS3.