Deleting a file doesn’t actually erase it from your computer; it only hides it from view. To a computer, “delete” just means that the space in memory is now free to use the next time you create a new file. When that space is used again, the old file is overwritten. But in the meantime, the deleted file is still there, and it can be accessed by someone who knows how to pull it directly from your computer’s hard drive. Wiping your device protects against that.
A wipe actively writes over your old files so that your old data is replaced, and thus removed. In a typical Windows or Mac “restore”, the wipe writes over your files once. There are software that will go further and write over your files several times, thoroughly destroying them; however, the instructions for installing and running this software are complex. For most of us, it’s probably not worth the effort unless you work for the DOD or NSA. And if you work for them, you don’t need our help wiping your hard drive.