Protective casings, such as jewel cases or paper sleeves, prevent CDs from gathering dust or scratches that can render them unusable. It is advisable to store discs in jewel cases when not in use, because this protects the CDs, reducing the risk of data loss.
Discs often become damaged as they collect dust and receive tiny scratches and abrasions while outside of their protective casings. Although discs can sometimes be repaired through cleaning and polishing, this is not guaranteed to work.
CD repair guides, such as the one found at HowToCleanStuff.net, recommend that users store discs in protective casings as a best practice to avoid damage. However, users should also make digital backup copies of CDs that contain important data so the data is not irretrievably lost if a disc is damaged beyond repair.