Answer:
to tell us we need one another to succeed
Explanation:
Anna Bradstreet was influenced heavily by growing up in a Puritan society. She was even considered to be controversial by some due to her behavior in the aforementioned society, as she sometimes did not want to conform.
I'd say the answer is that they 'add variety to the sentence structure'. Because you cannot write independent clauses the whole time, you will have to use a subordinate one at some point, it is inevitable.