In general, it's not necessary to double space a novel. However, if you think it'll make your writing and revision process more organized and clear, you can do it. Just keep in mind that publishers might have their own formatting preferences for the final submission.
Whether to double space a novel is a personal choice. Some find it helpful for visually distinguishing paragraphs and making notes. But ultimately, the focus should be on the quality of the content and adhering to the submission guidelines if you aim to get it published.