It's important to cite your sources. Using someone else's idea without giving them credit is considered plagiarism and academic dishonesty. This can have serious consequences.
Fortunately, it's really easy to avoid. Cite your sources. Cite all of them - not just textual, but any images, graphs, videos, audio recordings, or data you use.
One reason college students are taught this is because it's expected in the professional publishing world too. So, it's advisable to think critically and pay attention to citations when you are evaluating the research of others.