I'll give you my personal reasons for liking GWT in a few short bullets.
1. Better tooling - IDE refactoring when I need to change things. Red squigglies when I make a typo or reference something thats not there. A real debugger with breakpoints for finding bugs. Auto completion and auto style formatting so I dont have to type so goddamn much.
2. Compiler optimizations - Your code gets faster and smaller. Write maintainable code, have it turned into performant code.
3. API's and libraries - easy client server RPC, discoverable wrappers for the DOM APIs. Reuseable widgets that have predictable structure/side effects!
4. Maintainable code base - If you worked with more than 4 engineers on a non-trivial product, you would understand the appeal of a Java code base. UIBinder lets me not worry about CSS name collisions.
I would go on, but you seem like a TLDR type of person, so it would probably be a waste of my time.