1. Cross-browser compatibility.
2. Code performance.
3. Code organization (there are many JavaScript frameworks whose selling point have been making JavaScript more organized).
But in regards to this article, one often overlooked feature of GWT is that dead-code is automatically removed from the compiled JavaScript:
https://www.quora.com/How-fast-is-GWT-compared-to-JavaScript...
Thus you don't have to include the entire Angular/React/Ember/JQuery whatever library just to get a few features for your site.