I actually added that to the FAQ not long ago, because I agree it’s been unclear:
We use your real name for 1) the credits, if it’s the name you’d like to be credited under, and 2) our private tester records. Usernames and email addresses can change more frequently, so it’s easiest for us to keep track if we have another name as well. We don’t need your legal name if you prefer not to use it - a name you’re comfortable with is fine.