Why does iPhone games include a sound option? It seems very unnecessary because the sound options (on/off, volume) are already on the hardware itself. Unless the player wants to shut out music, or specific effects it seems like a useless setting. But still there are tons of games that only have on/off and volume setting, is there a particular reason for that?
One reason to support a software version of some option (such as volume control) when a hardware option exists is simply to provide users the choice: they can disable sound, or adjust the volume of sounds, for just the game while leaving other more important device sounds unaffected.
It's also possible that a game can just expose a software control to the same system volume that is controlled by hardware buttons.
Often such features are not difficult to implement, especially if they already needed to be implemented on another platform with fewer hardware controls, or if they were provided by the game engine being used. The cost to provide the feature is thus relatively small compared to the "quality of life" improvement seen by some subset of the player base.