Howcome the get/set in dependency property doesn't do anything?

I've created a dependency property like this:

public partial class MyControl: UserControl
{
   //...

   public static DependencyProperty XyzProperty = DependencyProperty.Register("Xyz",typeof (string),typeof (MyControl),new PropertyMetadata(default(string)));

   public string Xyz
   {
       get { return (string) GetValue(XyzProperty ); }
       set { SetValue(XyzProperty , value); }            
   }

   //...
}

Then bind it to my wpf window and everything worked fine.

When I tried to add some logic to the setter I notice it wasn't being called. I modify the get;Set up to a point now they look like this:

 get{return null;}
 set{}

And it is still works! How come? What's the use of that GetValue/SetValue calls?