Instance Properties and Variables

I am confused why some classes declare a property, but do not declare ivar and vice versa.

Is this standard practice when declaring an instance variable also declares it as a property?

Example:

@interface AppDelegate : NSObject <UIApplicationDelegate>
{
    UIWindow *window;
    UINavigationController *navigationController;
}

@property (nonatomic, retain) IBOutlet UIWindow *window;
@property (nonatomic, retain) IBOutlet UINavigationController *navigationController;

is standard practice when declaring an ivar class to make it also a property?

I get that @property creates its own elongated setter (and getter with @synethesize), but why should it also be an ivar?