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.
The problem was that if you used a third-party client, Reddit would have to coordinate with them to launch whatever new stupid cryptocurrency scam they wanted to push that week. On a web browser they can just push new code into it[0], and their first-party mobile clients can be updated ahead-of-time with support for the feature. But third-party clients would have to spend their own development time adding stupid "click here to get your Snoovatar[1]" links. They could slow-walk that, or just not implement that, and Reddit would have to spend time and money kicking users off that third-party app.
This, incidentally, is why every other major social media platform bans third-party clients. Third-party clients are user agents, not platform agents.
[0] Which, incidentally, makes web browsers not user agents
[1] An NFT scam Reddit tried to pull