Why might a writer choose to paraphrase a source

Sometimes so that way they don't have to source a million sources. They also do that so they have evidence of something without exactly quotating it from somewhere else.

When source material is too long and detailed to be quoted

Explanation:

When a writer paraphrases, he or she rewrites the information in his or her own words instead of quoting directly, trying to maintain the same level of details than the original text.

When summarizing, the writer captures the essential idea of a passage in a more concise manner. Thus, it is useful when working with long texts in which details are not so important to the overall idea.