Craftsmen and artists need to learn to use their tools. MS Word is a complex piece of software and the writer who uses it will benefit from purchasing a book or two and actually learning how to use it.
MS Word seems to randomly change formatting when you move things around in the document, whether you're writing a book or anything else. Using styles can save you from that. If you use Word, you can avoid many problems by using styles.
You're wise to understand ~ before you begin your project using Word ~ what's going on paragraph-by-paragraph, and the best way to do that is by using Word's styles.
You'll find a pretty good tutorial site that approaches using Word from a writer's perspective at Daiya Mitchell's website, "So You Want to Write a Book with MS Word."
An advantage to using styles: if you're going to self-publish ebooks, epub and mobi formats they behave much more nicely if you feed them a document that's properly formatted using Word styles.