You have one field on the user registration but under the hood it is used to fill both username and email (this way they are the same).
For upgraded projects (or users created with API - e.g. importing users from other system or using custom registration widget) you will have the ability to have different values for username and email. That's the reason the login widget allows using email or username.
If you setup brand new Sitefinity 10.0 and use the standard widgets - the new users will use email only.