Why some applications are used with double tap instead of single tap on my Nokia C5-03 device?

Menu and applications in Nokia C5-03 are mainly used with a single tap or a long press (a press and hold). Some applications still have a double tap functionality, e.g. Search. Also many third party applications are used with a double tap. Long press on selected item opens new menu with more options, e.g. in Music Player or in Contacts you can delete, rename, mark item etc.