Scholars place a premium on careful, original thought. Academic writing is essentially an ongoing conversation among scholars. As a college student, you are part of the community of scholars who are working to answer genuine questions in their fields by building on the knowledge and ideas that others have contributed. When you use sources to write a paper, you have the responsibility to represent others' ideas accurately and to contribute your own ideas to the discussion. Your professors expect you to do your own thinking, and they assign research and writing so that you can figure out what you think rather than reporting or parroting someone else's thoughts. You actually don't learn anything when you take your ideas from someone else; you learn by analyzing the ideas you have read about and developing your own responses to them.
When you conduct research for your own paper, you rely on the citations provided by other authors to find material relevant to your topic or question. Similarly, when you write an essay, you are responsible for providing a roadmap for your readers back to your source material so that they, too, can follow the conversation. By citing your sources you provide guidance to the scholars who come after you. In this way, anyone who wants to enter the conversation, to take a position different from yours, or to build on knowledge or ideas in your paper can do so.
So, for example, if you're asked to write a literature review for a psychology paper, part of your task is to provide clear information for your readers about where you found the literature you're reviewing. Similarly, if your Expos assignment asks you to read and critique Stanley Milgram's report of his famous obedience experiments and you come up with an argument that is informed by C.D. Herrera's (2001) claim that the deception in the experiments did not cause the subjects to suffer, then you must cite Herrera's article in your paper (Herrera, C.D. (2001) Ethics, deception, and 'those Milgram experiments.' Journal of Applied Philosophy, 18(3), 245-256.) and explain how his argument influenced your own thinking. The bottom line is this: Whenever you report on or summarize someone else's ideas, you owe it to that person to properly credit him for his work. You also owe your readers a roadmap through the literature you surveyed so they can see what led you to your conclusions and whether they agree with those conclusions.