Most people don't write reviews, and I can understand why. If a book, film or anything else already has a ton of reviews and you agree with the average rating it can seem pretty pointless. At the same time, writing a first review (or a second/third when you disagree entirely with the first) can be a bit tricky. After all, everyone's opinion differs and it can be quite difficult to write a good review that's objective enough to be critical when necessary without going overboard.
However, I do think that from both sides of the fence writing reviews is a good thing.
From a reader's perspective
Readers, or prospective readers, are pulled towards a book by a number of things, and these vary quite a bit from one potential reader to the next. There's the cover, of course, the summary of the plot, the sample, the overall rating and the reviews themselves.
The first of these is done by an artist, who will usually be a different person to the author. The next two are done by the author, but the advantage of the average rating and reviews is that they're written by people with no vested interest in the book. They also take less time to read than the 15,000 word (or however long it is) free sample.
So, they're more impartial and can be digested in a minute or so. This makes it easier to assess whether or not a book's worth a shot before downloading the sample (or buying it outright) and spending several hours on it.
For the individual writing a review, it can also be interesting to see whether or not one agrees or disagrees with reviews that have gone before. Was the comedy laid on too thick or was it hilarious? Was the whip-wielding dominatrix a bit of a cliché or rather thrilling? And it's fine to disagree. Books are incredibly subjective, and whilst disagreeing with someone does not make them wrong neither does it make one's own opinion invalid or dilute the value of it.
Writing a first review means you start with a blank canvas, but that's perhaps the best place to start. Review number 137 won't affect the rating much, and if a book has that many reviews already it's almost certainly sold a ton and one more review will neither help nor hinder it. But review number 1 can help. Or not, if the book is woeful and the reason nobody's reviewed it yet is because they're embarrassed about how awful it is.
From a writer's perspective
Writing is, by its very nature, a solitary pursuit. With the probable exception of the cover you don't need anyone else to be involved. Agents, publishers, editors are now optional extras, and even if you've gone down the traditional route then 99% of your time is still spent solo. The characters, plot, pacing, humour, grittiness, death count, word count and speed of progress are all down to you.
Lots of writers employ beta readers (objective people they know who can tell them X is good and Y is so appalling the author should be beaten to death with their own shoes), but this occurs during the process. Reading reviews written by others means that the author can enjoy the glowing praise of this and that, and also learn from the criticism of bits and pieces that don't quite work. As with so many things in life, praise is more delightful to receive, but criticisms often lead to the biggest improvements.
In short, reviews are useful for both potential readers and for authors, and it's a good thing to both write and read them. So, if you've read a cracking book that has no review to its name or a dire creation that should be sealed in a filing cabinet in a disused toilet with a 'Beware of the leopard' sign on the door, why not write a review?