With the correct software installed they will speed up Photoshop, specifically in opening, saving, changing the file format and compressing files within photoshop. Back in the day these were being made and used, Photoshop was horribly slow when manipulating, opening and saving large high resolution images. I think a lot of people forget just how slow the processors were and just how horribly expensive even moderate amounts of RAM was - and Photoshop gobbles RAM like there's no tomorrow.
Just to put things into perspective I personally (through the company I was working for) purchased some 16Mb SIMMS for a Apple Macintosh Quadra 950 for the specific purpose of speeding up Photoshop (or in this instance making it actually useable for practical photographic retouching) and each 16Mb SIMM cost about £600, that's just under $1,000 for one 16Mb SIMM. Now work out how much it would have cost to put 16 x 16Mb SIMMS into that machine.
The DSP accelerator card and software to go with it was relatively cheap compared to installing so much RAM and any speed boost you could get when running photoshop was vital given how hungry Photoshop was in terms of resources.
Erm, what else do you want to do with a DSP card designed specifically to speed up Photoshop?