Why would you have to use headphones in the field?
Well, honestly, if you can isolate the sound downstream of the preamps in a REALLY loud show, it can be a little bit nice to be able to hear what's being laid down on tape to confirm that nothing is being overloaded, such that if you have a choice you could dial in a pad or something.
One of the nice features of the SD-7XX series recorders is that the headphone amp is pretty decent and provides a nice volume level AND you can choose where in the chain you're monitoring the sound...such as from the mics or from the preamps. Another nifty feature of the SD product is that you can delay the sound to the phones by a bit so that you can differentiate the sound that's headphone sound from the actual show.
Again, though this is a headphone connection on the all-in-one recorder and I'd say that the major use of this is simply playback monitoring, rather than field monitoring.