We cite our sources when we are using someone else's words and/or ideas. It does not matter if we are paraphrasing another person, we still need to cite from where we are paraphrasing. Another reason to cite your sources is to give proper credit to the creator of those words/ideas. You wouldn't want someone to steal your creative output, artwork, words, or ideas. The last reason is so the reader can trace the original source or resource of the words and/or ideas.
We cite our sources when you are stating something that is new (not common knowledge like "there are 24 hours in a day"), something that you saw in a documentary, or that you read somewhere. Citing a source means you are giving credit to the creator of the idea and/or words.
You must cite your sources when:
When in doubt, cite the source.