Why use professional book marketing services?

I've been pondering something lately: why exactly do authors turn to professional book marketing services? It seems like there's a lot you can do yourself these days, but I'm guessing a good book marketing company brings something special to the table. Is it just about saving time, or is there more to it?

I'm thinking they probably have a deeper understanding of the market, right? Like, knowing which social media platforms really work for different genres, or how to get those coveted book reviews. Maybe they have connections with reviewers and influencers that we wouldn't normally have access to?

I'd love to hear from those of you who've used these services. What was your experience like? Did you see a significant difference in your book's visibility or sales? What were the biggest benefits you noticed? Or if you decided against it, what made you go that route?

Also, for those who've worked with a book marketing company, how did you decide on which company to hire? What should authors be looking for?