Is Hiring a Professional Editor Really Necessary for Every Author?

I’ve been debating whether to hire a professional editor for my manuscript, but I’m not sure if it’s always worth the investment. Can a professional editor truly transform a book’s quality, or can dedicated self-editing and peer feedback achieve similar results? For authors who’ve worked with one, did it make a noticeable difference in readability, flow, or market success? I’m curious whether this is an essential step or just optional for most writers.