Why.  Named icons automatically adapt to theme changes, giving your application a much more integrated appearance.

Since GTK+ 2.6, named icons can be used for window icons (see gtk_window_set_icon_name()) and images (see gtk_image_set_icon_name()). In GTK+ 2.8, you can also use named icons for drag-and-drop (see gtk_drag_source_set_icon_name()) and in treeview cells (see the icon_name property of GtkCellRendererPixbuf).