The older kindles with 2G/3G "Whispersync" functionality had another major drawcard of working just about anywhere, even if you didn't have wifi coverage or a phone with data services. I have in the past used this to load several more books while in an airport in a foreign country between flights.
Many people if you hand them a Kindle can easily navigate the store and buy a book.
Without that built-in store, then they're now going to have a bunch more hurdles: they need a computer, internet access (or an offline store), the skill and ability to run and use Calibre, the knowledge of where to get eBooks from, how to get them in the right format (or convert them), etc.