It's for emphasis.
"Tu" isnt needed because we only use "eres" with "tu" and in Spanish you can skip it.
Its like if someone says "am going now..." It sounds a little funny in English, but there is no doubt he means himself. You dont say he "am." Only I am, so he must mean "i" even if he didnt say it.
"Tu" is added in later to emphasize it - its like how we might say "I, myself, prefer the green ones." We already knew that "i" meant you were talking about yourself; you were just emphasizing
You can say "eres tú" if you personally know that person or even are friends with him. Or if you don't know each other but both of you are relatively young, because it becomes increasingly common to use the more personal addressing if you're around the same age.
You can say "es Usted" in all other cases: unfamiliar, older, more traditionally talking people.
This gives me Once Upon a Time vibes. Sorry, I just watched the finale..