It's as ease as 1 - 2 - 3!
You know the problem, you are developing a WPF application and you want to see how your XAML looks like. Normally you have to compile, start and see if everything looks like you want to. With FastWPF this process is getting speed up because it will show you instant the result within Visual Studio during your development. It is not trying to do the design work for you, but it specializes at showing you a preview of your XAML. A preview which you can interact with, so you can check a CheckBox for example, enter text into a TextBox and you see how it will look like. If you define complex, dynamic parts within your XAML for instance with some Triggers or converters you even see the reaction which will happen if you change the state of the user interface.
In case you are developing with the MVVM-pattern FastWPF will improve your daily development dramatically because you can design your DataContext like you want it to be. Just set up the type of DataContext you want to use, and afterwards you can even design your DataContext. If you want to can add some items to a bound data collection, you can change every single entry of the collection and see how your application will look like with your design data. Every DataContext you are designing is stored for later use.
Support for Resources
If you are developing an Application with XAML you will have at least one ResourceDictionary, with FastWPF you can look at every ResourceDictionary you have (App.xaml, Generic.xaml or a custom one) and you can see the keys and values of it. So it is even possible to see how you are designing your Controls, Styles or Brushes.