Too many writers (and non-writers) we've worked with over the years fail to use appropriate or effective topic sentences. As you may recall from seventh grade grammar, a topic sentence tells readers what to expect from a paragraph. While the topic sentence doesn't have to be the first sentence in a paragraph, many readers expect it to be and take that first sentence as their "cue" in terms of what the paragraph will be about. When we're talking about scientific and technical writing, writing a topic sentence is crucial.
Effective topic sentences should encapsulate what the paragraph is about and should, if at all possible, indicate how the topic will be approached. So, for instance, I can write a topic sentence that says "The FDA is reviewing various databases" or I can write a topic sentence that says "The FDA is reviewing various databases for the purpose of understanding how certain drugs are used in children." As you can see, the second sentence provides us with a better idea of what we can expect in the paragraph. By cueing in the reader in terms of the topic and how I'm tackling it, the reader can decide whether to read the paragraph in its entirety or skip it. And, an effective topic sentence can also help writers stay on track. A strong topic sentence helps guide the writer when writing the paragraph.
Finally, writing a topic sentence that indicates what the paragraph is about and how the issue is being handled ensures "flow" (that is, logic). To create flow, I simply need to ensure that one or more of the keywords I've used in that topic sentence (and by keywords, we mean those words that actually tell the story of the paragraph) are used throughout the paragraph. Of course, we also need to ensure that the topic sentences create a pattern throughout the document, but that's a blog left for another day.