From what I've read up through google (being a comp e major I'm a bit interested), yeah most earphones that don't have high input impedance don't really need an amp, and probably won't benefit much from one. I think the issue is that some devices don't have enough power to fully drive whatever earphones you're using, so maybe some low sounds and whatnot may not properly make it through the input impedance to the transducer.
Also, I'm nowhere near an expert on this side of things (I prefer digital circuitry to analog circuitry and all of its wonderful Laplace transforms), so feel free to correct me... but this is also another thing that I'm wondering about: what happens when you turn up the volume to a level where the earphones respond well enough for you to hear what you want (ie you hear it at the volume you want), on any normal device? There
has to be an increase in power somewhere, and that's in the device you're using. But what if sound reproduction on the device gets worse as you turn volume up? In other words, more distortion, clipping, etc. in the sound as you jack the volume up to the levels you want. This device would be fine powering some crappy iPhone earphones or something because you could probably keep the volume relatively low and be fine (granted the quality wouldn't be that good anyway with those earphones), but when using your higher quality earphones you need to turn the volume up to hear everything well.
I would guess this is where an earphone amp would come in, because you could keep the volume the same on your device, but the amplifier would make the output sufficient for your earphones, with minimal distortion. Heck, it might include signal processing units to filter out noise and whatnot, too. This would be a lot better than increasing the volume on your poor iTouch or whatever you've got. My guess is that the sound would be much more clear, too, because the device is outputting at a comfortable range.