When you write, you need a point. That point should be clearly stated in your thesis statement in your introductory paragraph. In one stunning sentence, you should be able to state what you are arguing and why.
You might have started with a strong thesis statement and then found that your body paragraphs are straying from the original argument. If you keep an outline next to you (or open in another screen) as you write, you can make sure that you stay on target. I like to call the outline a “cheat sheet.” If your thesis is written on the top – maybe even in bold – then you can’t forget what you are trying to prove.
Composition writing students always ask me why need to have a thesis if they are writing a research paper. My answer is that you’ve gathered research on a subject for a reason. It is the support for your claim. Your claim is your thesis. Therefore, you are proving something and need to make it clear to your readers.
If you’ve written body paragraphs that don’t quite connect to your thesis, you could prompt yourself by asking, “so what?” Sure, this might sound slightly rude at first, but it is helpful. If a fact/line/piece of research isn’t relevant and you can’t answer the question, “so what?,” then you probably don’t need the information.
I would argue that any piece of writing – poem to a screenplay – needs to have a thesis statement, but that’s for another post.
For more information about thesis statements, here is a great link from UNC.