It is a useful instrument for investigating the scope of the proposed paper in order to determine if it is too broad or too narrow. It is not intended to remain fixed, but can be adapted throughout the process of writing the paper as new arguments are discovered or old arguments are deemed to be irrelevant.
The thesis statement is often used in an adapted form at the beginning of the finished paper. It is considered to be better writing to not introduce the thesis statement with the words "The purpose of this paper is to..." but to reword it to make clear what will be following, and thus it is often found after a brief motivation for writing the paper.
In general, a thesis statement is a selection that explains what your document is going to be about. It also contains brief reasons why you chose this subject. For example, if you were to be doing a paper about bringing someone from the past to the present, it would most likely look like this: "The person I chose, if I could bring someone back from the past to the present would be...". You would also add the reasons which might look like this: "The reason I chose (person's name) was because..." You would add multiple reasons to your thesis statement. This however is not a thesis statement for upscale papers.