Why "they" is translated as "die"?
You can do that to avoid confusion between Sie = you and sie = they. In many contexts, a demonstrative pronoun sounds more natural than a personal pronoun.
But still, "die" would not be necessary here, right? There's no Sie/you in the sentence, so no need to disambiguate.
It's not necessary, but it's not wrong, either. In spoken German, you can't hear the difference between "Sie" and "sie". So it makes sense to use "die".