This is the 1773rd most frequent Russian word.
зачем
"Зачем" means "why" or "for what purpose" in English.
In this sentence, 'зачем' is used as an interrogative adverb to inquire about the purpose or reason for the early arrival.
Зачем ты пришёл так рано?
Why did you come so early?
Here, 'зачем' poses a reflective inquiry into the rationale behind requiring something.
Ты сам должен понимать, зачем тебе это нужно.
You must understand yourself why you need this.
In this case, 'зачем' is utilized within a subordinate clause to convey uncertainty towards the purpose of an action.