the speaker in a headphone cup works as follows: current flowing through a coil of wire that is fastened to the speaker cone generates a magnetic field, which interacts with the magnetic field of a fixed magnet, moving the cone and so generating sound.
the cone movement, and hence volume, is proportional to the strength of the magnetic field, which is, in turn, proportional to the current through the wire and the number of turns in the coil.
there is a tension here, which is not obvious. the problem is that the coil is fastened to the cone, and so moves with it. for a responsive cone, we want as light a coil as possible (a heavy coil will give the speaker cone inertia, making it less faithful to the music signal). but if we make the coil lighter by reducing the number of turns we reduce the volume; if we make it lighter by making the wire thinner we increase the resistance, lower the current, and again reduce the volume.
one way to work around this is to supply a higher voltage to the coil. that can counteract the higher resistance of a thinner wire, and so provide the same volume with, hopefully, better sound quality. but the output voltage of many sources (particularly portable players) is limited.
so, finally, all should be clear: people use amplifiers like the one here to increase the available voltage so that they can use higher impedance, and arguably better sounding, headphones.
the cone movement, and hence volume, is proportional to the strength of the magnetic field, which is, in turn, proportional to the current through the wire and the number of turns in the coil.
there is a tension here, which is not obvious. the problem is that the coil is fastened to the cone, and so moves with it. for a responsive cone, we want as light a coil as possible (a heavy coil will give the speaker cone inertia, making it less faithful to the music signal). but if we make the coil lighter by reducing the number of turns we reduce the volume; if we make it lighter by making the wire thinner we increase the resistance, lower the current, and again reduce the volume.
one way to work around this is to supply a higher voltage to the coil. that can counteract the higher resistance of a thinner wire, and so provide the same volume with, hopefully, better sound quality. but the output voltage of many sources (particularly portable players) is limited.
so, finally, all should be clear: people use amplifiers like the one here to increase the available voltage so that they can use higher impedance, and arguably better sounding, headphones.