They don't have that FPGA to offload the CPU, I think its rather to interface with external components. GPUs don't come with external IO and for a lot of high-speed buses you are just dead in the water without an FPGA to handle the delicate timing.
That is really the unique benefit of FPGAs: the implicit parallelism makes them a really great fit for the kind of high-speed bit banging that would be impossible to get right on a normal CPU (not to mention very difficult to program in the first place).
That is really the unique benefit of FPGAs: the implicit parallelism makes them a really great fit for the kind of high-speed bit banging that would be impossible to get right on a normal CPU (not to mention very difficult to program in the first place).