What purpose does a 'topic sentence' serve in writing?

The purpose of a topic sentence in writing is to express the main idea of a paragraph. It serves as a guide for the reader, summarizing the content that will be discussed in that specific paragraph. A well-crafted topic sentence clearly indicates the focus of the paragraph, helping to unify the supporting details and examples that follow. This clarity is crucial for effective communication, as it allows readers to understand the central point being made without ambiguity. By establishing the primary idea at the beginning, a topic sentence helps to set the tone and direction of the paragraph, making it easier for readers to follow the writer's thoughts.