Storage media, such as DVDs and CDs, offer no guarantee that the digital information stored on them will remain accessible.
Firstly, the longevity of the storage media is determined by its quality, and secondly by the availability (or lack) of hardware needed to read the media - consider for example the drives needed to read floppy disks. Furthermore, the software needed to read and correctly interpret the files quickly becomes obsolete.
Periodic transfer of stored data onto another, more contemporary, storage media is required to ensure that In information is not lost due to these reasons.
In addition to periodic refreshment of the physical storage media, it is also important to ensure that media are correctly handled so that they can best withstand the tests of time.