In practice
Content tools use it to suggest terms competitors cover that you do not. Read those as topic gaps, not as words to insert.
Never write to a TF-IDF target; use it to decide what a page should discuss, then write it properly.
Why it matters
It is a useful way to see coverage gaps objectively rather than by intuition.
It is also routinely oversold as a ranking formula, which produces mechanical, unreadable pages.