To acknowledge or give credit to the person whose work you quoted, paraphrased, or summarized
To allow readers to find your sources
To provide evidence to support your argument
To avoid plagiarizing
To let your professor know that you did your research
To participate in the academic conversation on your topic
When to Cite...
Cite your source when you are quoting, paraphrasing, or just referencing any of the following:
Anyone else's articulated ideas, arguments, opinions, or experiences.
Any artwork, pictures, videos, or other creative works produced by others.
Direct quotations of any words written or spoken by others.
Unique phrases or terms coined by others.
Data, statistics, or facts produced or documented by others.
Published research details and results, whether conducted by you or others.
When NOT to Cite...
According to the Purdue OWL, you do not need to cite for purposes of credit when:
You are writing about your own lived experiences, your own observations and insights, your own thoughts, and your own conclusions about a subject
You are writing up your own results obtained through lab or field experiments
You use your own artwork, digital photographs, video, audio, etc.
You are using "common knowledge". This can be a little tricky, but a general rule of thumb is: If the majority of people already know the information, then it is "common knowledge".
You are using generally-accepted facts, e.g., pollution is bad for the environment, including facts that are accepted within particular discourse communities, e.g., in the field of composition studies, "writing is a process" is a generally-accepted fact.
You find the same information not documented in five or more authoritative sources.