The primary reason to backup data is to avoid data loss.
There are many ways to lose information from a computer - a power surge, theft of the computer itself, lightning, floods, and sometimes equipment just fails. If you regularly make backup copies of your files and keep them in a separate place, you can get some, if not all, of your information back if something happens to the originals on your computer.
What is a backup?
In information technology, backup refers to making copies of data so that these additional copies may be used to restore the original after a data loss event. These additional copies are typically called "backups."
Backups are invaluable following a disaster (called disaster recovery), or to restore files after they have been accidentally deleted or corrupted, or as a result of data loss.
What to backup and how often?
Anything that cannot be replaced easily should be backed up. The question of 'how often' depends on individual circumstances - the currency of the data; the ability to replace it, if at all; the cost of replacement including the 'cost' of time.
Where to backup?
A basic rule for backups is multiple copies in multiple locations on different media.
Student backup options - recommendations and facilities for backing up course related data
Staff backup options - recommendations and facilities for backing up work related data
On your home computer, consider backing up the following:
Bank records and other financial information
Software you purchased and downloaded from the Internet
Music you purchased and downloaded from the Internet
Your e-mail address book
Your web browser bookmarks