In our world of instant communication, reviews have become important. Writers, publishers, producers, agents, and other readers pay careful attention to what reviewers say or don't say.
A review is somewhat like a tip at a restaurant. It isn't necessary. A tip isn't even expected in fast food places. Writers certainly don't expect reviews from all their writers, but a carefully thought-out review is a tip in the sense of a suggestion. It's a chance for the reader to leave a tip about what was good and what could be improved.
A review doesn't need to be long; however, a carefully crafted, accurate review can be a work of literary art within itself.
A review should tell the truth in kind words. It shouldn't contain exaggerations.
A review should reflect good etiquette in the sense that the reviewer is tactful. Maybe some writer has worked for years on a book. An inconsiderate reviewer, who ignores all the good in a book, can virtually shred it with unkind remarks.
A review should not be placed after a work for the purpose of destroying the author.