Whether you are self-published or published through an agency, it is extremely important that you gain as many honest reviews on your book as possible for a few main reasons.
One of the main reasons why you need to gain reviews on your book/s is for your own reputation. Building a reputation as an author can be challenging, as you can not really make a social media presence for a writing process. The best way to build your reputation is through reviews of your book. These reviews will help you to gain a following, it will help more people become interested in your work.
Another reason why you need reviews on your books, more so if you are self-published through Amazon, is so people can understand what your book is like. When purchasing books online people like to see what other people are thinking about a book, this can help them to decide if the purchase is worth their money or not.