This controller is simply a slight evolution of the original Genesis controller. Three additional face buttons have been added, for a total of 6. A mode button was also added in order to enable 6-button mode in 6-button compatible games.
Sega's original Genesis controller was probably looking a little out-of-date compared to the Super Nintendo controller, which had more buttons so Sega released a revised controller.
Unfortunately, the new controller came pretty late in the console's life and most people still had the original controllers. Few games were made to make use of the extra buttons. The button configuration was also a bit awkward - six buttons to one thumb. Remembering what each button was supposed to do in games wasn't intuitive as there was little to differentiate one button from another.