Reddit wanted to control how users consumed content on their site. To control the experience (i.e. monetize with ads), they had to shut down third-party clients, since those could remove ads.
Google appears to be doing the same thing, but for the entire web. WEI is a way for sites that want to monetize with Google ads to prevent folks from accessing their site unless they can cryptographically assure that the user's browser will follow all the rules Google sets. We don't yet know exactly what all those rules will be, but it isn't hard to guess that they'll be along the lines of whatever makes Google the most money.
This applies to desktop browsers, but also affects automated tools like wget and curl. It could kill web scraping altogether.