As an End in Itself
Most of the summaries that we write to remind ourselves of articles we've read serve only that purpose. For a short project or paper that uses only five to ten sources, a nutshell statement or précis for each source may suffice. For a longer project (more sources or extending over a long time), more detailed summaries help writers remember sources accurately.
Teachers may also assign summaries as ends in themselves when the articles are complex and teachers need to be sure that everyone understands the points in the assigned readings. Or teachers may assign summaries to help students practice writing accurately and concisely about the subject matter. Sometimes these summaries serve to introduce students to jargon or concepts particular to that discipline.
Finally, summaries are sometimes written as separate pieces of reference works. Typically called abstracts, these summaries help readers decide if they need/want to read an entire article.