It also gives them significant power over authors (and therefore publishers). If you know that a huge portion of demand via Amazon purchases (where many people will purchase your book) will be correlated to the goodreads review, you're going to optimize for that platform. I've seen this before with friends who are new authors that send out their books and their biggest ask is to give it five stars and a good review on goodreads... which drives more traffic to Amazon and creates a flywheel for them. Plus, the direct integration after you read a book on Kindle to quickly give a book a star rating means that you want your Kindle version to be optimized/prioritized not just because of the market of Kindle readers, but because it'll send more traffic to feed into your rankings and therefore affect your demand more.