Most of the time when you delete a file, it’s actually not wiped from existence immediately — as recently discovered by a man whose wife came across illicit texts he’d been sending (and which he thought he had got rid of on his iPhone). The man is now suing Apple.
Marital infidelity aside, the safety net that’s put in place for deleted files can help you recover data you’ve erased in error — however, it can also leave your data exposed to other people (if you’re selling a device, for example). It’s important to know where your deleted files are and how to get them back or erase them permanently.