The diffuser is kind of building block for making reverb. It only runs when you've got a Digidesign card with a 56k DSP on it, like AudioMedia or ProTools, in addition to the SampleCell.
I got an AudioMedia II partially to use it, and I can't say I've run it much. I've not really got stuck in with it, but it doesn't seem to do anything interesting to the sound. Maybe if you run a couple in parallel with some delays etc. maybe you could make a tinny sounding reverb. It's not some amazing high-end sample mangler, unfortunately.
Very much optional, though in my case it being a single card means I can run ProTools alongside the SamleCell in a Quadra 700, for four-track hard disk recording. Pretty sweet.