Optical media are probably the most secure backup media. Recordable CDs and DVDs are sturdier than tape or hard drives. The data stored on them is not affected by electrical or magnetic disruptions and they have higher tolerance for heat. Estimates for the life of such media can extend into the decades with minimal proper care.
That does not mean that there aren’t limitations of optical media. They are slower than fixed media (and, depending on the specific technology, also slower than tape media). They also have distinct capacity limits, which are much lower than fixed media and tape. And despite the fact that there are rewriteable CDs and DVDs, they are not as readily reusable as fixed media. These limitations mean that for server and network backups, optical media are generally not considered viable options. However, optical media are well suited to data archiving. If you are removing data from your network, but will still need to retain a permanent record of it (data pertaining to graduating student work or archive copies of workstation image files, for example), placing that data on a CD or DVD where it can be easily accessed if needed and where degradation is not a concern is an excellent option.