Commonly appearing at (or near) the beginning of a paragraph, a topic sentence expresses the main idea of a paragraph. (thoughtco.com)
Preview Skimming In Action Research indicates that more than 50 % of the time, the main idea of a paragraph is in its first sentence. (marzanocenter.com)
Plus, it allows you to answer those «What was the main idea of paragraph 1?» (thoughtco.com)