When writing a report or even a form of correspondence, you always need to introduce a new topic in a paragraph with a topic sentence. The purpose of this sentence is to prepare the reader for the point you wish to make.
According to the Purdue University Online Writing Lab (2009),
“Every paragraph should include a topic sentence that identifies the main idea of the paragraph. [It] also states the point the writer wishes to make about that subject. Generally, the topic sentence appears at the beginning of the paragraph. It is often the paragraph’s very first sentence. A paragraph’s topic sentence must be general enough to express the paragraph’s overall subject. But it should be specific enough that the reader can understand the paragraph’s main subject and point.”
The key function of a topic sentence is to improve the overall readability of a paragraph and ultimately the entire text.