a REST API doesn't necessarily mean outside access. For one thing, it’s a quick conduit for React and Angular front ends. I've had countless requests for this kind of thing — in fact, this API makes a hacky plugin of my own thankfully obsolete.

Beyond existing sites, the REST API means that Wordpress can be used as a quick-and-dirty back end to datasources that are not necessarily bloggy. Folks use Wordpress for things beyond blogs because they don't want to build custom CMS's, now this CMS has a whole new field of possibilities it can fulfill.