The reason that students and faculty need to cite is very simple and foundational to academia, in that it is our chosen method of communication.
Proper citation is integral to the transfer of ideas and concepts in a responsible and effective manner where both the creator of a work and the user of a work are both fairly given credit for their respective ideas.
Citation allows us to communicate in an enriching way and further the scholarly discussion. Improper citation devalues our scholarly discussion and the scholarly work done at our institution.
You need to include a citation whenever your work contains or refers to someone else's:
In-line citations or foot/endnotes (depending on the assigned citation style) should be inserted at the point-of-use in your assignment and indicate that the information you just presented came from a source other than your own brain or common knowledge.
A bibliography and/or works cited section should be included at the end of your assignment.
There are hundreds of bibliographic styles, each with its own precise formatting. The most popular at CapU are MLA and APA.