The iPhone and iPod Touch are great for playing music while you gaze at the screen. They're also great for playing music while the screen is off: save power! extend battery life! The catch is that the typical way of stopping the music is so cumbersome. If the screen is off, that means the device is locked. So you turn it on, do the "slide to unlock" thing, and then hit the Pause button.
When the (non-iPhone) phone rings, or someone walks into your office to talk to you, or your speakers start blaring some inappropriate song, or you need to dash out of your car... that's just not fast enough.
Here's the shortcut: Unplug the audio cord from the iPhone or iPod. Even though the internal speaker could keep playing a tinny version of the same song, the device detects the unplugging and pauses the music automatically.