The answer is: Because if you don’t, then this mistake will bite you, sooner or later.
Why? Because of Murphy’s Law:
Anything that can go wrong, will go wrong.
And because a variation of it, Finagle’s law:
Anything that can go wrong, will—at the worst possible moment.
The worst case scenario is that all your (and other’s) data will be gone, forever. And if you are operating a web server, in addition to that hundrets of hours of your time configuring it.
You are running RAID? This is good, but not enough. What if your entire machine is hit by electric discharge (or similar desasters), destroying basically all electronics? Then RAID will not rescue you in this case.
The only reliable backup solution is to have one or several external copies, separated from your main machine. Preferrably you have physical access to the hard drive containing all data.
But even if you do this, there are more caveats:
If you are using rsync to update your backup, and you accidently delete files on the original, rsync will also delete these files on your backup! So, to be uber-cautious, you should have a history of backups.
Do you have a method in place with which you can quickly restore the backup? No? Then write scripts for it, now.
Don’t say I haven’t warned you. And in case you didn’t do your homework and all is lost, don’t blame it on ‘evil’ external circumstances, because:
Never attribute to malice that which is adequately explained by stupidity.
What if all data are still lost?
Well, in this case I only can say that
Every misfortune is a blessing in disguise.