For example, if I have a WordPress site I've built from scratch out of WordPress.org, I am just going to assume trying to put it on WordPress.com will be annoying (and possibly even impossible?), because of issues with themes or plugins or whatever due to the fact that WordPress.com is a separate, hosted SaaS-style CMS, and not a hosting environment for WordPress sites.
WPE, by contrast, is Just Another Webhost to me, with some special bells and whistles for WordPress.