Yep, as Dawn says they do it to aid digestion. I'm glad you mentioned it as I knew that cats did it but didn't know dogs did. A friend at work was saying that her dog had sicked up grass and we wondered if dogs did it for the same reasons as cats.
I use chick crumb too, I had a petsitter for my cats in the summer and when I got back she said to me 'Oh, I didn't realise you used Worlds Best cat litter - I didn't flush though as you'd specifically mentioned binning the waste'. She couldn't believe it when I told her it was chick crumb, which incidentally you can flush and is around £10 for a huge 25kg sack.