Why do websites use “email” and “username” interchangeably as log-in credentials?

Question

Why do websites use “email” and “username” interchangeably as log-in credentials?

In: 58

Answer 1

Theoretically it’s a world wide unique value, unlike a local to the system username (where you might have a “Tom” username in every other system in the world). You are the only one with your e-mail address. So the user can use the same identifier that they use everywhere else. While “Tom” might be taken in half of the systems you use.