Because if you don’t you can lose 13 years of work…
http://news.bbc.co.uk/2/hi/technology/8049780.stm
Some malicious hacker got into the servers used by avsim.com. The person in charge of the 13 year project forgot to back stuff up. Well not exactly forgot, he just had two servers which cloned themselves to one another. Which means when the malicious hacker got access to one server he was able to get access to the second server and destroy the data on both of them.
Always, always, ALWAYS and once again ALWAYS have off network backups. And if at all possible have offsite backups as well. That way you have a chance of recovering after a catastrophe such as the one in the article.