Book reviews are additional sales. This is how people will get to know about the existence of your book and what it’s about.
The reviewer engages with the book, tears its arguments, lauds it’s successes and notes it’s shortcomings. The end goal is an informative and honest review but with utmost respect to the author, and publisher.
Books are my happy place and I’d genuinely wish to review every book I read to support every author but there’s the ‘have to pay bills ‘but’; there’s the ‘this is a job but’; there’s the ‘time but’.