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If you're deaf in one ear, then listening to any music that's a stereo mix can be frustrating; you might be missing half the orchestration or the vocals. When you turn on the Mono Audio option in Settings→General→Accessibility, the iPhone mixes everything down so that the left and right channels contain the same monaural playback. Now you can hear the entire mix in one ear.
Tip:
This is also a great feature when you're sharing an earbud with a friend, or when one of your earbuds is broken.