People love reading a good book and when they do, they usually like to tell others about it. They like to share their find with others who they feel would enjoy the book as well. Word of mouth does a lot for book sales, but leaving reviews on online book-seller sites is also important.
If someone wants to buy a book, they may be swayed by the number (or the lack) of reviews on Amazon, Barnes & Noble, or Chapters/Indigo. The lack of reviews may make them wait and although they may come back later for a book, there's always the chance that they won't.
So, if you like a book, let's say like Fits, Starts & Matters of the Heart (no pressure!), you can help the authors by leaving your reviews on any or all of those sites. You don't have to have bought your book from the site to leave a comment.