While hard disks are fairly reliable they can go wrong and you can lose all the data on them. They can also of course be stolen.
The only true safe backup is to put it on CD or DVD (two copies just in case).
You ought to think to yourself, what if my laptop / hard disk got stolen, or it crashed or whatever (worst case scenario).
In an ideal world all your important files should be backed up on to a series of CDs or DVDs, if they are not then you leave yourself open to losing the lot.
I read a story about a chinese man in the UK who had about 9,000 digital photos on his laptop from loads of family holidays. None were backed up anywhere else.
His laptop was stolen and he lost the lot. He was offering a reward, not for the return of the laptop, but of the photos.
Often the data on the PC or laptop is more valuable than the PC or laptop itself.