connect your writing with that of others on the topic (a scholarly conversation)
give texture to your writing
show that you "did your homework" and researched your topic and provide evidence to support your idea
Do not cite sources to make a point for you. Always make sure to include your own analysis and point of view.
When must you credit sources
You must give credit to your source when
you are using words or ideas presented in a book, journal, magazine, blog, website, movie, TV program, or any other medium
you copy exact words
you are paraphrasing. This is where a lot of unintentional plagiarism happens. MLA9, 4.7 tells us how to paraphrase
you reprint any diagrams or charts
when you reuse or repost any digital media
There are more situations, see "Plagiarism FAQs." Purdue Online Writing Lab and MLA9 4.4-4.11.
When you don't need to give credit
You don't need to give credit when
you are writing your own experiences, observations, or insights
you use your own artwork, videos, photographs, audio, etc.
you are using things that are common knowledge
you are using generally accepted facts, including those that are generally accepted in the discipline in which you are writing
you mention an author or work in passing (4.14)
when you allude to a well-known passage for rhetorical effect (4.15)
if you have an epigraph, which is a short quotation at the beginning of a work that establishes the mood or theme (4.16)
MLA9 defines common knowledge as basic biographical facts about prominent persons and the dates and circumstances of major historical events (4.13). The Purdue OWL says for something to be common knowledge, at least 5 credible sources must have the information without citation.
Remember, when in doubt, cite your source! Better to cite and not need to than to plagiarize. Unintentional plagiarism is still plagiarism.