That's a good question! Before I got cuttelbone I had no idea what it did until it looked into it. Like @Deanasue said, it adds calcium for the snails to harden and grow their shells.
I just broke it up and put a small piece in at a time. Like 2 sugar cubes worth. Cuttlebone will cause PH to rise a little. That’s why I just put a little in at a time.