For cats older than three months, you also need a claw board.
Cats often lick their bodies, so fur gets into their stomachs.
This fur forms a ball inside the stomach so the cat needs to vomit up the fur ball. Eating grass helps them to do this. Cats that are kept indoors do not have access to grass. Pet shops sell grass seeds, which you can plant inside your house and let your cat eat. There aren't any pet shops in the vicinity of my home.