Just like deleted data on your computer is not really deleted, deleted data on your phone is not really gone. The phone is essentially a mini computer, and operates in same way as any other computer you own. When you delete data, the path to the data is deleted. The data is still present in the same spot on the phone's memory, but the phone thinks that it is not. When space is needed to write new data, those "empty" spaces that are created by removing the path to the data is used - or overwritten - by the new data.
A good example of this is a phone that we are currently working on. I'm not going to provide you any in depth details, because we value our customer's privacy as much as our customers do, but here is the basics. The phone was factory reset; no "old" data appeared to still be present on the phone. Multiple actions were completed on the phone after the reset, including setting up a new account, connecting to the internet, downloading files, and more. Despite all this, we were able to recover extensive messaging and call history, contacts, 100s of pictures and videos, and even audio files!
What sort of data was saved on your old smartphones? Or your children's smartphones - things you never saw because your child deleted them! Sure, you could attempt to ensure no one else can see anything saved on your phone by smashing it to pieces before you throw it away, but even that doesn't ensure that the memory within the phone is cannot be recovered. Even when a phone doesn't work (broken screen, won't power on, etc), the memory inside the phone may still be hanging on to the things you saved there once upon a time.
Before you pack away your old smartphone and forget where it is, sell it for some extra cash, or even throw it in the trash, let us help you make sure your data has been securely removed and cannot be recovered. Mention you read this blog, and we'll offer you a limited time special: $99 + shipping and handling. You'll never have to worry about your messages, emails, pictures, or *gasp!* videos falling into the wrong hands.