The quickest way to confuse a reader is to set out the elements of the law and then fail to address each in turn. Because the contents of the paragraph usually focus on a particular legal element, your topic sentence should refer to that element specifically. Topic sentences are also extremely useful in helping a reader focus quickly on a particular piece of the analysis. Think of the occasions when you have tried to develop just one facet of your argument and have searched for the analysis on that point in a case or legal article. It is very frustrating to spend unnecessary time searching for something you know is there but cannot find easily.
Topic sentences can cure that problem.
A clear topic sentence that identifies the argument being made in the paragraph helps readers focus on the parts of the document they find most relevant. After you have finished your draft, create a reader-based outline developed from the topic sentences in the document. Then step into the shoes of a reader who knows nothing about this area of the law and read the result. If you as the reader find that you have a coherent understanding of the law based on this outline, then you know that the large-scale organization of your document has presented the analysis clearly and efficiently. Only then do they apply the law to the facts at hand.