In a CD or DVD, the data is stored in pits on the surface of the disk. However, rather than each pit corresponding to a bit, the transition from pit to no-pit or vice versa, or lack of, is used to store information.
However, does this have any advantages over each pit corresponding to one bit? I can't seem to find anything about it online.
I suspect it may have to do with writing to the disk, or perhaps with reducing the damage caused by scratches on the disk, but I haven't read anything that has any reasoning for why.