Some games need more buttons that a gamepad can handle, or some games need a button that is used only once in the game. You can add “extra buttons” that will be displayed on the screen, when pressed they can send a key or a gamepad button.
For example, the game Rescue on Fractalus for Atari need many buttons, it’s a game designed to be used with a keyboard. Although you there is a gamepad mapping for this game, it’s easier to play using extra buttons containing the real action names.
This is the extra button definition for Rescue on Fractalus
Land = BTN_A Fire = BTN_B Sys = BTN_X Ship = BTN_Y TH- = BTN_TL TH+ = BTN_TR Door = BTN_TL2
And this is how the buttons look in the game screen: