When it doesn’t flow. When the words are tangled and don’t make sense. When I can’t tell my “that” from my “which”, when my apostrophes have taken on a life of their own and words have run amok I turn to this ‘quiet little book’.
Rule Seventeen. Omit needless words!
Vigorous writing is concise. A sentence should contain no unnecessary words, a paragraph no unnecessary sentences, for the same reason that a drawing should have no unnecessary lines and a machine no unnecessary parts. This requires not that the writer make all sentences short or avoid all detail and treat subjects only in outline, but that every word tell all.
William Strunk JR. The Elements Of Style