What sells best? Word of mouth. Comments add a product confidence factors for visitors. If I am hesitating between two products, and product A has received one good comment (or one good rating), but product B has 10 ratings, 7 of which are good, I will buy product B due to the trustworthiness given by the fact that 7 different people have tried it and approved it. There are even techniques to automate this analysis process.
As an e-commerce store user, there is a factor that devaluates comments to a very high degree: misspellings. Amazon has an entire team that corrects misspelt comments. Just this makes comments more serious, which makes us trust them more.
As for Ratings, they are a basic tool which every e-commerce store should have for its products, as it’s the first thing you see: starts, numerical scores… anything that shows the product quality/acceptance by customers who already purchased it.
In addition, these tools can be used to obtain patterns-purchase relationships between products and users. For example, if I gave the films The Lord of the Rings and Thor 4 stars, and Pepe gave 4 stars to The Lord of the Rings only, we probably have similar tastes, and it would be a good idea to recommend Thor to John next time he enters the store. This is known as a “collaborative filtering”, and can be used for cross-sale, for example.
The first screen capture in the Ratings comes from www.imdb.com, which isn’t an e-commerce store, but I think the way in which they summarise the ratings for series/films is every interesting.