Here's why:
* Clarity: Starting with the main point helps readers quickly understand the paragraph's focus.
* Organization: It creates a clear structure and guides the reader through the supporting details.
* Signposting: It acts as a signpost, indicating what the paragraph is about.
However, it's important to note that topic sentences can sometimes be found:
* At the end of a paragraph: This can create a sense of suspense or emphasis, but it's less common.
* Within the paragraph: This is usually done to create a more subtle introduction or to build up to the main point.
Ultimately, the best placement of a topic sentence depends on the specific paragraph and the desired effect.