Where is the purpose expressed in an essay?
in the thesis
It's basically the reason why you did what you did. For example, in a English test, if they ask "what is the expressed purpose" of why the author wrote the story, they are asking "why did they write it?" I asked my English teacher what "expressed purpose" meant, she just told me "Why did they do it!"
The author's thesis is usually a statement that the author will spend the rest of the essay trying to persuade or prove to the reader how and why the thesis is valid. The author's purpose is the rest of the essay to prove the thesis statement. The author is trying to persuade the reader that the thesis is correct and valid and that is the purpose of the author's essay.