In WPF i can make my UI using .NET 2.0 style like designer.cs, my question is what are the advantages of using XAML for UI design instead of code.
The main advantage is that if you keep a clean separation of markup and code (e.g. by applying the MVVM pattern), you can concentrate on the behavior of the application (i.e. the code) and let professional graphic designers worry about applying the look and feel of the application.
These two tasks can even be worked on in parallel by two different people using different tools (e.g. Visual Studio vs. Expression).