When you enjoy a good book, a simple rating or even a review are appreciated by the author as much as your continued reading. This is a simple way of showing your appreciation for the entertainment that the writer provided. Doing so helps promote the author's work to other readers. It's simple to click the stars. We all strive for 5, but we know we can't please everyone.
Adding a review is a bonus. Reviews can provide your thoughts on what worked in a story or what didn't. They can be one sentence or several paragraphs, although it is considered a courtesy to include a big SPOILER ALERT heading if you giveaway key details that readers may not want to know in advance. Many readers know this, but there are always new readers reviewing their first books in their enthusiasm who may not.
I enjoy product reviews for my own evaluations before buying, but in books, I don't like to know too much. And when it comes to stories, what one person finds distasteful may be candy to another. I look for certain details in stories, just as others do. Some details are helpful, but you have to use your best judgment of what to tell potential readers.
Please consider a rating (and possibly reviewing) after finishing a book, especially one that you enjoyed. Other readers appreciate this, and so do authors. It's a simple way to say "Thank you!"