One function of the thesis is to inform the reader of the topic and of the argument the writer is trying to make. It also gives the reader clues as to how to read the thesis. Because the thesis statement clearly states the argument, the reader then reads the thesis critically, seeking out the main points of the argument. The thesis statement tells the reader what to look out for as he reads through the paper.
The thesis also functions as a tool for the writer to help her to focus the scope of her research. Facing a library full of books can be intimidating for even the most seasoned researcher. It's easy to be steered off the course of your research plan, but if you have a clearly defined thesis statement, you'll know where to begin your research and how to continue. In order to keep your research focused, a tight thesis is an invaluable tool.
In some cases, you may find that your thesis is too broad and that your research is becoming unfocused. In these cases, the function of the thesis is a research aid, helping you to see that you bit off more than you can chew. It is now time to refine your thesis and make the scope of your argument more specific.
Once your research is complete, the thesis becomes a writing tool, helping you to organize your research and present the arguments in an order that makes sense to the reader. Answering the question or problem your thesis poses in an organic order should be a natural extension of the thesis. The thesis should also function as a guide as to how your research should be used, whether to compare and contrast findings or to analyze information gleaned in the research process.