I learned this lesson the hard way. In my final year at University, my HDD went with no warning and I lost everything. Offcourse I didn't have any backups either. I would have failed my degree if it had not have been for one data recovery company who helped me out for a few hundred pounds. Others were quoting thousands! The company was called CBL tech if anyone is interested.
Since then I've had another HDD fail but thankfully I had full backups of everything. It's amazing how much important data you just don't realise you need until it's gone.