Are Book Editing Services Really Worth the Investment?

I’ve been considering hiring book editing services, but I’m not sure if they’re truly necessary. Do these services genuinely improve a manuscript’s quality, or can careful self-editing achieve the same results? For authors who have used them, was the investment worth it in terms of clarity, readability, and overall polish? I’m trying to decide whether professional editing is essential or just an optional luxury. What has your experience been like?