Citing your sources gives authority to what you have to say
Do you know everything there is to know about protozoa? What about post-impressionist art? Do you know how to take a tractor apart and put it back together again?
Even if you were an expert in any of these fields, you wouldn't know absolutely everything about them. Who could? In the information age every subject area contains more information than any one person can learn in ten lifetimes.
When an expert writes a paper they cite the original works they used as sources. Citing the sources demonstrates that they are familiar with, or even building on, the work that others have done.
Demonstrating that you have read what the experts have to say gives weight to your work. It also allows your instructor to look at the sources you used to further their understanding of the topic, as well as to evaluate your understanding of it.