Buckley (2004), in her popular writing text Fit to Print, defines the role of summarizing as reducing text to one-third or one-quarter its original size, clearly articulating the author’s meaning, and retaining main ideas.
When summarizing is useful
Summarizing is useful in many types of writing, at different points in the writing process, and at different spots in a written essay or other piece of academic writing. The benefit of providing a snapshot of main points lies in offering the reader a "big picture" that allows the reader to contextualize what you are saying. For example, you can summarize:
In addition to the advantages of summarizing for the reader, as a writer you gain a better sense of where you are going with your writing, which parts need elaboration, and whether or not you have comprehended the information you have collected.
Buckley, J. (2004). Fit to Print: The Canadian Student’s Guide to Essay Writing. (6th ed.) Toronto: Nelson.