Whether someone needs developmental editing depends on the person and their manuscript and their goals for the manuscript. If you’re just writing because it makes you happy and you don’t care about making money, or because you just want to share your story with your family, maybe you don’t need it. If you’re creating a book out of a course (which is already structured well and pretty much acts as a detailed outline), maybe you don’t need it. If you prefer to use beta readers and critique partners to help you get the organization down, maybe you don’t need it.

It's totally up to you, your goals, your budget, and your timeline.