If you choose to publish only on KDP, your ebook will be available only on Amazon.
Amazon’s share of the global ebook market is high, but on the long term, it is worth working with many stores, just to have more feet on the ground. Being available in multiple countries, on multiple devices is definitely worth it.
According to our current sales numbers, Amazon contributes to 45% of the global ebook sales, while 55% happens elsewhere. We strongly advise you to publish your ebook in many other stores, such as Apple iBooks or Google Play and on new, subscription based platforms, such as Scribd.
The more stores your books appear in, the higher your SEO results and Google ranking will be, which can result in higher sales numbers.