Classes defined as not being descended from GLib.Object are treated as a special case. They are derived directly from GLib's type system and therefore much lighter in weight. In a more recent Vala compiler, one can also implement interfaces, signals and properties with these classes.
One obvious case of using these non-Object classes stays in the GLib bindings. Because GLib is at a lower level than GObject, most classes defined in the binding are of this kind. Also, as mentioned before, the lighter weight of non-object classes make them useful in many practical situations (e.g. the Vala compiler itself). However the detailed usage of non-Object classes are outside the scope of this tutorial. Be aware that these classes are fundamentally different from structs.