The ease of use is great! You can easily add your products to their shopping cart through e-mail or website links without having them log into an account first (which I dislike).
It's easy for me as well because it gives detailed information about my customers' buying habits so that i know where they're coming from in order to market more effectively towards those people who are interested in what we offer. They have helped us increase revenue by several hundred dollars per month with little effort required since adding our product line was relatively simple compared to other platforms out there like Facebook ads etc..