Performing regular backups should be considered one of a responsible system administrator's top priorities. Although Linux is an extremely reliable operating system, failures can, do, and probably will occur. They may be caused by hardware failure, power outages, or other unforeseen problems.
More likely will be those problems caused by human error, resulting in undesired changes to, or even deletions of, crucial files. If you are hosting users on your system, you will most certainly be requested to restore an inadvertently deleted file or two.
If you perform regular backups, preferably on a daily basis (at least for user files which are updated often), you will hopefully reduce the possibility of, and increase your recovery from, such file lossage.
The safest method of doing backups is to record them on separate media, such as tape, removable drive, writeable CD, etc., and then store your backup sets in a location separate from your Linux system. Sometimes this may not be practical -- perhaps you do not have a fire-proof vault in which you can store your backup tapes! Or perhaps you do not have access to such an external backup system in the first place. Nonetheless, backups can still be performed, albeit on a slightly limited basis.
At my place of employment, I perform backups on several Linux servers. Depending on the situation, some of these backup sets are written to tapes, others are written to a separate server over the network, while still others are simply written to a separate disk partition (for example, in the ``/archive/'' file system) by an automatic cron job (perhaps because the server is in a remote location, for which a daily visit to perform a tape backup is impractical or impossible).
At home, I do not have an external backup system, nor do I have massive amounts of available disk space to write a backup image. Therefore, I instead back up only my user files on ``/home/'' as well as some customized configuration files in ``/etc/'', writing the backup set to a separate disk partition.