Some folks read about "good reviews" when they see a book or a movie advertised and maybe think, "so what?" They may think that it is only there to boost an author's ego or sell more books.
Honestly, it does both of those things, but there is a larger purpose than creating the need for a bigger hat. Here's why it is important to you as a reader:
When you rate a book you've read on Amazon, Goodreads, or a publisher's website, you've told other readers whether the material is worth their time to read. They do the same for you. Would you rather read a four or five star-rated book, or a one star?
Usually, the higher ratings give you an indication whether you'd buy it, true. Perhaps just as importantly, that rating tells the author how well the material is being received in the readers' world and whether or not to adjust their styles, genre, storylines, etc. That gives you, the reader, a better selection to choose from eventually.
When you ask, "Where are all the good books?" you should be seeing stars and they will guide you to them. I hope you will consider rating each book you read.