Whenever you use other writers' ideas, however, you must acknowledge your sources. Doing so allows you to distinguish between your ideas and those of others; it directs your readers to relevant sources; and it allows you to give credit where credit is due. It is important to properly reference and cite your sources to avoid plagiarism and to give credit for original ideas.
Putting source material in your own words shows readers that you have a true understanding of that material. Also, to restate in your own words the full meaning of a phrase or passage helps readers understand difficult, complex, jargon-riddled or ambiguous passages.