Your publisher will likely submit your book for review to some people and magazines. Just because they do doesn’t mean everyone will review it. It also doesn’t mean every review posted will be a positive one. That’s the nature of book reviews.
So why bother?
Because people notice. Maybe some people read online reviews. Maybe they don’t. But most people notice whether a book has 0, 15, or 400 book reviews. And that tells a potential reader something right away. It tells them whether or not people are reading the book.