Wiping data is not the same as deleting data. Deleting data only changes the master boot record entry for a file (the index used to find parts of the file on the hard drive); the file (or some of its parts) remains on the hard drive until new data is written over that part of the hard drive. This is how digital forensics experts are able to reconstruct deleted data. Wiping, on the other hand, securely overwrites the data in the file with random data. On solid-state drives, wiping is very difficult, so I carry a laptop that has a standard hard drive and wipe it with at least thirty-five passes.
