As I understand it the idea is that your email address is your identity so you only need to remember that (which most people are capable of) rather than having to a) remember your username/password (which can at least be stored in LastPass, etc for you) or b) having to remember which one of the social login buttons to click on.
If you signed up with a yahoo email address for example, it would (I think) let you login with a yahoo account instead.
