Paraphrasing a source enables the author to includes their own original ideas after a citation while still abiding the factual statements provided by the source. This technique consists on rephrasing ideas or information from an original source, while still providing credit to the respective authors.
An author might choose to paraphrase in order to properly introduce a source of information before using it in their document, instead of simply stamping the information and thus avoiding making the fragment look like a copy-paste.