Re: Why unity and coherence are so important when writing an essay?
Unity is when all the parts of each paragraph lead toward the same main point (as established in that
paragraph's topic sentence), while coherence is when all the parts of a paper "flow" well together. the ideas in one paragraph are related to the ideas in the next and the next, in a clear and logical way.
Both of these things help a reader figure out what you are trying to say in your paper. To this end, unity makes sure the reader doesn't get lost (or inundated with irrelevant information) within a paragraph, so they can stay focused on your point. It is a lot easier to understand what you are trying to say if the reader is not distracted. Similarly, coherence makes sure the reader can't easily loose the thread of the paper, between paragraphs and overall. Together, they make the whole paper make sense to the reader.