[Mirioron]

I don't understand why people do that. Your 2nd authentication factor should not be something relying on the same device that you're using.

[roywiggins]

It reduces the set of people who can access your account from "people with the password" to "people with the password and access to my phone."

It's less like a 2nd factor and more like a poor man's password-protected private key authentication, but it's way better than just a password.