Is a Book Proofreading Service Really Necessary?

I’m debating whether to hire a book proofreading service for my manuscript. Do professional proofreaders really make that much of a difference compared to self-editing? How do you know which services are reliable and experienced in different genres? Are there common mistakes authors make when choosing a proofreading service? I’d love to hear from writers who’ve used these services did it improve your book significantly, and was it worth the investment?