Putting ideas from a source into your own words is a good way to be sure you have understood the source's meaning and to show this understanding to your reader(s) / teacher(s).
This is also helpful when taking notes and rephrasing test questions.
You do not have to paraphrase sentence by sentence. You can paraphrase larger pieces of a paragraph in order to express an important idea or ideas.
Watch this 4-minute video that explains why paraphrasing is important.
Warn, M. [Michelle Warn]. (2015, July 23). Ashford learning resource library: Paraphrasing - How to use other's ideas in your writing [Video]. YouTube. https://www.youtube.com/watch?v=jyCMZRWo9Tg