Instead of downloading email attachments and opening separate media programs, you can simply view the file from inside the app.
Again, while it's not offering anything you can't do on email, the experience is simpler and more immediate.
Nobody is saying you can't send emails to friends on your mobile phone, but it's fair to say that there's a place for both types of services.
If you just want to send a quick message or photo and would appreciate a swift response, these apps are definitely worth exploring.
Apple has really shaken up a lot of the core apps with i OS 10 as well as drastic changes to the Lock Screen.