Imagine that you have a lot of hard work done, and are rather pleased with your accomplishments. Then suddenly, in a moment of technological fail, everything that you did erased right then.
It's happened to many of us, and it could very well happen to you. Backing up those important projects is essential to making sure that you're not frustrated when hours of hard work get flushed down the drain.
If you back up and something happens, you can restore what you've done. If nothing happens, well it least you did it.
We only have so much time in the day, don't leave yourself open to do what you've already done. For some reason, it's worse the second time, mainly because you've felt like all of your accomplishments got flushed down the toile