Commas are used to sort of rope off dependent clauses that, although they add to the sentence, aren't imperative to the sentence. You can remove any phrase in a sentence that is corded off by commas and the sentence should still be complete and readable. Yes, I know there are a few other ways to use commas, don't get me wrong.

When you add the comma in front of "respectively," it makes sense because the sentence can stand alone without the addition of that word.