I personally don't use them... and I don't think I've ever seen them mentioned before on this site.. I suppose they could be used to lower the amount of nitrates produced. Anybody use them?
They are not a good idea IMO. I believe that if you do have an ammonia problem, then there is something wrong with your tank that needs to be fixed and the ammo chips only fix that for a little while.
I use them when transporting fish, or when i have to put them in a holding tank for some reason (like when rearranging my living room.. or when we recarpeted, etc) whenever they're stress level is increased and filtration is decreased.
But as far as during normal filtration, I don't use it.