"She works hard every day."
都 refers to each single day, where the action is performed.
很 heightens the extreme of the adjective. In the sentence 很努力 refers to "very hard work", which is being worded differently in the resulting sentence.
I agree. They did state in the tips and notes at one of the first few units that they'd be requiring it as a general rule, and I understand their desire for uniformity (here), but I think this sometimes does more harm than good, especially where the sentence flows naturally without it.