The Council of Writing Program Administrators gives the following definition:
"In an instructional setting, plagiarism occurs when a writer deliberately uses someone else’s language, ideas, or other original (not common-knowledge) material without acknowledging its source" (WPA, 2009).
You give credibility to your own work when you let others know where you got your work.
It helps others learn more (taking a "deep dive") about a particular aspect of your topic.
You are spreading the word about other people's hard work.
It's the ethical thing to do.