1. Reviews can improve my book's placement so others will see it. If your book is not on the first few pages, it won't be seen. Readers typically only scan the first few pages when they are searching for a book.
2. Reviews can help get a book on lists like Hot New Releases.
3. People won't buy books if they don't have reviews.
4. A review can convince a reader to either get the book or pass on it.
5. Reviews can let the other know what the readers like or didn't like about their book. That helps with future writing projects.
There are other reasons, but these are the top ones. If you love a book or even just really like it, please consider leaving a review for that author. More reviews mean I can spend more time writing and less time marketing. Plus, I love to hear what people think of my books. Writing is such a solitary job. It's nice to get feedback.