There are a few concepts that help explain the importance of citation in assignments:
When you do research on a topic, you are engaging with others' research and ideas through articles, lectures, interviews, etc. Often that research and those ideas are also built on the work of others who are engaged in creating new knowledge in this field of study. By citing your sources, you are providing the thread that connect various participants in, and contributing to, a broader continually evolving conversation.
It is important to give credit to others' for their ideas. Intellectual property has both legal and social elements that are defined by the context in which it is used. In the context of higher education in North America, others' information is valued and should be properly attributed as a principle of academic honesty.