Word of mouth still matters. It matters a lot, to pretty much to every writer who a) isn’t famous, and b) doesn’t have a major publisher willing to spend a fortune on marketing. Even for those, it still damn well matters. For someone like me, with a debut short story collection out, and a debut novel on the way, word of mouth is everything.
But what does this term word of mouth, mean exactly? Well, making words with your mouth hole, to start with. The oldest of old school methods. People still listen to friends, especially if those friends have the consummate taste to enjoy my collection. 
The internet is a megaphone, of course, and as such reviews on Goodreads and Amazon are becoming more and more important. I’ve read it is as high as 90% who rely on online reviews before making a purchase. This is certainly true of books, where a potential reader can see very quickly, a) how many reviews (is it popular?), b) the average rating (is it good?), and c) the most popular reviews (why do people like or dislike it, and if so, does that gel with the reason the customer usually likes or dislikes a book?).
Sufficient reviews on Goodreads can also encourage local book sellers to stock the book. If I were to go to a bookstore and ask them to put my collection on their shelves, they in turn might check Goodreads to see if it is popular enough to bother with. If potential readers go and ask for a copy at the bookstore, this can prompt a manager to go online to see whether the work is garnering praise (I know this is true, because a reader did this, and their bookstore agreed to order my collection).
While it is true that Goodreads is owned by the global vampire squid of Amazon, it is also true that writers like me are dependent on these gleaming sepulchres of avarice. That’s the downside. The upside is that leaving a review is an easy to do. On Goodreads you don’t even have to write your mouth words down onto the screen – just click a star rating (and upvote reviews that say the kind of things you were thinking). If you’re kind enough to leave a review, a magnum opus on the glory of my work is certainly appreciated. But a handful of pithy words are fine (just make sure they’re extra pithy).
The final (not last, but I got the main ones here) reason to leave a review is the mythical algorithm. The theory being that the more reviews garnered, the more that particular work finds itself 
projected into the eyeline of prospective buyers through ‘recommended’ and ‘also reads’. This goes on long enough, and the virtuous cycle begins of reviews, sales, algorithm magic, and then more reviews.
How many reviews? Who the fuck knows. I’ve heard twenty in two months, I’ve heard fifty as the minimum, I’ve heard more. It’s guesswork because no-one really knows, probably not even the people who wrote the algorithm. But we do know this:
People do judge a book by its cover. The art, the title, and the blurb – especially if a famous writer is saying nice things about the work. We live in a world saturated by choice and oversaturated in opinion. We make decisions fast, yet can do so in a reliably informed way. If we like the cover, and the blurb, then usually we check Goodreads. If GR gives the go-ahead, then a purchase decision is made without reading a single word of the novel or the collection. 
I’ve got a pretty good title, good art, an effusive quote from Richard Morgan, and a fair few reviews. But on the last, I could always do with a few more and if you have the time – and especially if you enjoyed the collection – then please, go ahead and leave a review.
If you do, I’ll buy you a pony or a katana*. Your choice: pony or katana.
*Due to Amazon and Goodreads conditions, I can’t buy you an actual pony, or an actual katana. Wink.