If you look at the Audiomedia I and II Nubus sound cards you will find it does have an onboard DSP to do sound encoding, and all of the Nubus era Video editing systems also had DSPs. The problem you run into is stand alone cards are too slow for some functions because you are dumping the data back and forth over the Nubus BUS. The best DSP card for photoshop were the last Radius cards with the DSP on the card itself. The last AVID Nubus editing systems had a data cable tied directly from the capture card to the DSP card so they didn't have to use the Nubus BUS.

The very first Quickdraw accelerators were nothing more then a couple DSP's on a Nubus card with some RAM, and those died out quick because they were too slow (I have a couple Rasterops cards in use and they do speed things up, but also bog down the Nubus BUS).

Another early use for DSPs were JPEG compression and decompression cards, I think Picturepress was one of the apps that used them. Early 68K processors were not optimized for that kind of work. They also did a specialized DSP chip/card for HD data compression.

All in all they are fun cards to mess with on Nubus Macs, I have quite a few, but I would not spend too much money on them since they are very specialized.