It would eliminate someone being able to get your username or password via this method though. Because you never have to type them in.
With my MacBook I can use either touchid or my watch to automatically unlock it, so I don’t even have to type my password in to get into my laptop. And then I use touchid and Keychain for all my passwords so I never have to type those in either.
Uh Snapchat already does that. Nothing about it is encrypted. And deleted messages are only deleted from your device, not snapchats servers.