Simply put, not citing a source is unethical because you are passing someone else’s ideas and research off as your own. A citation acts as an indication of where ideas and information is coming from, so that your reader can retrace the steps you took in gathering your research if they are interested in learning more about your topic.

The University of North Carolina at Chapel Hill offers a useful analogy for thinking about using sources:

[In] the vast majority of assignments you’ll get in college, your instructors will ask you to read something (think of this material as the building blocks) and then write a paper in which you analyze one or more aspects of what you have read (think of this as the new structure you build). Essentially, your instructors are asking you to do three things:

Show that you have a clear understanding of the material you’ve read.

Refer to your sources to support the ideas you have developed.

Distinguish your analysis of what you’ve read from the authors’ analyses.

When you cite a source, you are using an expert’s ideas as proof or evidence of a new idea that you are trying to communicate to the reader.