A good example of their application is low end oscilloscopes. The heart of the machine is an FPGA running the sampling, DSP, and all the channels. It's very parallel, does a good amount of compute, and also provides lots of arbitrary IO. It's too expensive to spin an ASIC for such a low volume product. A serial processor is orders of magnitude too slow. Not familiar enough with DSP to know why not DSP, possibly too specialized.