Writers, unlike speakers, can't rely on nonverbal strategies to clarify their meaning. They must provide signs and guideposts to help readers follow the logic of their ideas. Writers must introduce information in the correct order, choosing topic sentences that signal and explain why each paragraph is important to the reader. Writers must modulate the length and complexity of each sentence and paragraph to illuminate the relationships among topics and ideas. Few people do this when they speak.
Writing is much more difficult, and typically requires revision.
I have revised this paragraph four times, and will likely do so again soon.