To permanently delete a file it is not enough to press delete or to empty the recycle bin. Operating systems like Windows or Mac OS delete the file by marking it invisible and by leaving the disk space it was using. In fact, it is impossible to delete the file, you can only overwrite it with some other data. This basic principle of computer file system is used to undelete files. If we have, for example, accidently deleted some file, we can recover it by using some undelete tool. If the data was not overwritten we can get the original file back as it was prior to deletion.
For many computer users this behavior of files is not problematic. We do not care what happens with the files after they are deleted. But there are cases when we care or at least we should care. When we would like to dump the old computer or donate it to someone we do not want to leave all the files on the disk. Deleting them or even formatting the disk does not solve the problem. With simple recovery tools it is possible to recover formatted partitions and deleted files. This is a problem because our files may contain confidential data, user names, passwords or personal data.
This is even a great problem for companies. They can not simply give away old computers containing sensitive documents with confidential data. Formatting the disk, of course, hides files only from amateurs. Therefore, some way to erase the data has to be found. And it was. All you have to do is to overwrite the disk with random or nonsense data. This data overwriting will prevent conventional data recovery methods. Only conventional because there are also unconventional or exotic methods that can examine the hard drive platters under electronic microscope to recover even overwritten data.
To erase a hard drive you can use one of many hard drive erasers. This is a software tool that overwrites selected files with predefined data patterns to prevent data recovery. To be on a safe side one pass is not enough. Most disk wiping algorithms approved by permissions use multiple overwriting passes to remove all traces of the original disk data.
Even if you intend to dump your old computer it is a good idea to permanently delete all the data stored on the hard drive. You never know who might be interested in recovering the files from an old computer. Some do it for fun, others for business.