Over time, small sections of data on any hard disk can become (silently) corrupted when bad sectors occur. A bad sector is a hardware failure of only a very small part of the disk. This means that even though you have carefully kept backups, and your backup disk appears to be working fine, some of this backed up data may be lost or corrupted without your knowledge. Only when you need to recover the data do you discover that some files have been corrupted and are unusable. You can also get corrupted backup data for other reasons, such as power outages, faulty disk firmware or RAM.
This is why it is a good idea to occasionally validate your backup data. Validation involves automatically checking all your backup data, at the lowest (bit by bit) level, to ensure none of your data has become corrupted over time, or during transfer. Since every single bit must be checked, validation is a slow process, but typically worth running now and again for piece of mind.