The thesis is the main argument you are making. By placing it explicitly, in the first sentence you are writing, you let the reader know exactly what your paper is about and where you plan to take it.
Your supporting arguments reinforce the thesis you are proposing. By briefly outlining what they are in the topic sentence, you alert the reader to what you will be elaborating in the next paragraphs.
In fiction, poetry or other creative writings, a topic sentence can powerfully introduce ideas, characters or emotions in just a few short words. Charles Dickens' opening to "A Tale of Two Cities" richly describes the struggle and rift between the title cities of London and Paris with the immortal words, "It was the best of times, it was the worst of times."
Concluding statements restate the explicit topic sentence, but at the end of the essay in the final paragraph. By restating the topic sentence, and supporting arguments, the thesis becomes much stronger to the reader.