Parenthetical elements are words, phrases and clauses that add detail to a sentence, but which are not “essential” to an understanding of the sentence.
A parenthetical element “could” be enclosed within parentheses, commas, or double dashes and left in the sentence, or it could be removed from the sentence, and the sentence would still be a thought fully expressed.
Uses of Parenthetical Elements:
There is no "law" that rigidly defines when to use parentheses or commas or a double dashes. It is up to the author to decide "how" he or she wants the reader to read the sentence. It is how a writer creates and adds tone and voice to a writing piece.
If it you want your reader to read the parenthetical element without emphasis, use commas.
If you want to whisper or say something as an aside, use parentheses.
If you want to shout or add a lot of emphasis use double dashes.