Many people may not realize this, when a file is "deleted" it is not actually removed, it is simply marked as free space but the data still exists. This can be problematic for a slew of reasons, corporations may have sensitive data sitting on a hard drive that can retrieved by someone who know how. Selling a computer to someone that has been previously used and may contain bank information or other personal information you do not want to leak out.
Retrieving the data is relatively easy, there are many programs out there that will do the job for you both paid and free. All a person needs to know is this process is possible, download the app, and run it to locate retrievable files on the drive in question. As stated previously this can be a problematic issue however, if you are running Windows 7 or later there is a built in function to help with this similar to the OS X wipe free space feature. If you want to wipe all of the free space on your Windows 7 or later computer so the data cannot be retrieve then simply run the following command, replace <drive> with the letter of the drive you want to wipe the free space on: