It is just awesome:
- Unit-Testing: though that picked up over the last few years, it's also way more mature in the Java world.
- Continuous Integration and Continuous Inspection
- Documentation Generation: Sure you have JSDoc and a few others
and allows you to determine what gets sent to the client arguably more easily than with equivalent full JS solutions.
It provides a good separation of concerns for large applications, with decent MVC or MVP architectures already pre-baked at your finger-tips.
GWT provides interesting libraries, and makes it easy (well, easier) to build I18N-enabled applications with dynamic bundle loading.
Using JUnit from within Eclipse IDE and from the command-line. This relates to my first point. You could very well also use some of Java's code quality tools on a GWT project (for source checks, not bytecode checks, as there isn't any).
It's All About YOU!!
If you want most of the above but you just don't want Java, maybe look at Google Closure, or the Dojo Toolkit.
Was a Good Idea at the Time: History Matters!!
With Regard to your Original (Now Edited) Question About the Use of Firebug
You can debug GWT code with Firebug of course, but ideally you would debug it directly from the Eclipse IDE's debugger, which now provides live code debugging support.
With Regard to your Original (Now Edited) Question About CSS
Yes, you still need to write CSS code yourself, of course. You couple your GWT project with other tools though (like SASS), more or less easily.
It's Just a Tool!