Apple has yet to provide us the ability to lock individual apps from prying eyes. We have our whole lives in our phone and everyone shouldn't have access to it when we share our phone or lose it.
The iOS jailbreak world has had the ability to lock individual apps for years now. Yes, the iOS devices allow you to add a security code or even a complex password to lock your device, but this not enough.
The all or nothing approach to security is not adequate. There are times when you hand your device to a friend to view pictures or even a text message. You don't want them roaming into your private pictures or email or even financial information.
Ever give your device to a kid to watch a movie or play a game only to find later that moved icons, deleted apps or even peeked at a SMS message that came in at the time? Haven't? Probably don't want to either.
Apple has made a small step towards securing your content and just your device. Guided Access does one thing; it locks the user into one app at a time.