When use [[UIApplication sharedApplication] delegate]

I don't know when or why I should use [[UIApplication sharedApplication] delegate]

I use delegate when [self.navigationController pushViewController:myView Animation:YES] does't navigate. I make MyView *delegate = [[UIApplication sharedApplication] delegate] and [delegate pushViewController:myView Animation:YES] to make the navigation work correctly.

I don't know if the solution is correct and in addition I don't know what is the use for [[UIApplication sharedApplication] delegate] or what are its effects on the application.

Can someone help me?

Thanks!!