Writers love to write and readers like to read. This is pretty obvious - right?
An author writes a book and either has it published by a third party, or the author self-publishes. Readers - especially fans of particular authors - purchase the book or download sometimes for free. What then?
How does the author know if a reader likes the book, loves the book or even (heaven forbid) hates the book? Unless a reader writes directly to the author the only way to tell if the book is doing well is by sales or more importantly - Reviews! A review which states "this is nice but..." allows the author to understand how the reader perceives the book and possibly what can be done to improve the story. A review which states "I love this book, when's the next one coming out?" makes writing the book really worthwhile.
Unfortunately, not many readers leave reviews. If only one in ten readers wrote down their thoughts on whether they enjoyed the book; which bits they liked; what they thought could make it better - all these responses would help the author improve their work.
If you read a book and enjoy it - take an extra few minutes and leave a review on your favorite online book store. You will make the author's day when they see you've shown appreciation of their work.