A clear topic sentence is important to both the reader and the writer. It helps the writer to stay within the limits of his chosen topic. This makes the process of developing the topic easier and ensures that you remain within the boundaries you have set without being too narrow or too general in your discussions.
To the reader of your essay, the topic sentence brings out the connection between one idea and the other. This is important in creating a logical flow from one idea to the other. In most cases, these ideas will be presented in different paragraphs. It is the topic sentence that links the ideas wherever they are placed within the paper.
A topic sentence should appear as early as possible in the body paragraph. It is usually the first or the second sentence in the paragraph. In every paragraph, it serves as a mini thesis statement because it sets the agenda within this paragraph. It is the defining and unifying factor in each paragraph. Without the sentence, such a paragraph would be hanging and lacking in direction.
The topic sentence is not always explicit in all essays. However, it is important since it lends relevance to the other sentences and content. It also gives the paragraph a position or context within the paper. This happens when it connects the current paragraph and its ideas to the previous paragraph. Such a sentence must relate closely with your overall thesis statement.
It is possible to trace the main ideas of any topic using these sentences. They are the anchors of each paragraph and provide a context for each idea. It is the sentences that create unity between different ideas and paragraphs. Since the sentences are related to your thesis statement, they keep your work focused on a singular idea as captured in the topic. This will ensure that your paper does not go off the topic.
The choices you make on the topic sentence will determine cohesion and focus of your essay.