Inspired by the recent question on list of figures: what is the point of ending your introduction with a paragraph saying "The paper is structured as follows: in Section 1 we do BLAH, then in Section 2 we do BLAH, we move on with BLAH in Section 3, and conclude with Section 4 in which we do BLAH"?
Many papers in my area do it, my coauthors add them to our papers, and I kinda took the habit, too, but I never really understood why. It looks like a poor man's table of contents. After all, we already write what we plan to do in the introduction, and remind the reader in the conclusions.
The answer is really the obvious one: it is to alert the reader to the structure of the paper. In addition to this part in the introduction, the paper should also, every now and again, tell the reader where they are in the overall story of the paper. This makes it easier for the reader to read the paper, by giving them the global picture and pointing out from time to time where they are in the global picture.
A table of contents would be overkill (and it would take up too much space).