It is needed for bone growth, interacts with vitamins, and is the substance that builds egg shells.
Deficiencies can cause poor growth, condition, and egg binding.
All my birds always have a cuttlebone near a perch. The cuttlebone is placed in a specially designed holder that has a perch attached. The mineral grit supplement contains oyster shell and minerals. I do add tiny amounts of activated carbon but its value or harm is still under debate. This cup is easily removed from its holder for easy refill.