The point is, if you read something you like, the best way you can help the author is to post a review. It doesn't have to be lengthy, but can just state what you like best about the book and what you didn't like if that applies.
Unfortunately, like every other field on the Internet, scams have developed. There are sites where authors can buy positive reviews and have them posted. But Amazon and others have been working to combat this by purchasing reviews for fake books and then searching their own sites for other items by that reviewer and removing them. And there are writers who actually open several accounts with different email addresses so they can put up several reviews of their own work.
So, the more real readers who post reviews, the more obvious these fake attempts will be. All ethical writers will thank you.