[anoobis]

Yes.

Okay, that may be a somewhat facetious answer; I'll expand on it. If you're using the headphone out of, for example, an iPOD, soundcard or stereo system, then that output has/is an amplifier. So, in that sense, you need one but you already have one. The real question is, 'Should I buy a dedicated headphone amplifier?'

This question is fraught with subjectivity surrounding sound quality and perceived value. Here are some thoughts on adding a dedicated headphone amplifier.
 

1. Your system has a line level output (only). For example, you have a DAC connected to a computer or you have RCA out from a CD player. Here, you need an amplifier as you cannot safely connect headphones to line out.

 
 

1. Your (portable) player will not drive your headphones loud enough. This is especially the case with high impedance headphones, where the player does not have enough voltage. A headphone amplifier should be able to overcome this.

 
 

1. Output impedance. An ideal output will have zero output impedance (barring the little-used 120 Ohm standard). This does not happen in practice. The greater the deviation from zero, the more likely distortion of the signal and tone. A headphone amplifier will present a very large load to the previous output; that output impedance is negligible in comparison and the amplifier sees a cleaner voltage signal than the headphones would if driven directly. Of course, the assumption is that the headphone amplifier has a low output impedance, and this should be the case with a dedicated device.

 
 
       It's worth highlighting that one of the effects of high output impedance can be to alter the tone. In the case above, the fabled 'wire with gain' amplifier will alter the sound but it is not imparting any colouration; it is preventing changes to the signal at the (original) signal-headphone interface. In this case, double-amping is a good thing!

       I do wonder whether manufacturers give any thought to the issues of output impedance and try to compensate in other ways to start with.
 

1. Battery life. Usually, the power required to drive the signal at an amplifier input will be less than that required to directly drive headphones. Therefore it may be preferable to use an amplifier with a portable source to prolong the time between charges.

 
 

1. IEMs. Many earphones/IEMs have low impedance and high sensitivity and are designed for use with portable (i.e. low voltage) players. As a result, there is no problem supplying enough voltage to drive them loudly. The current requirement will be higher, though I suspect that in most cases the battery should be able supply the required power. However, the low impedance does means that player output impedance can be an issue. So, headphone amplifiers for use with IEMs? Hopefully someone else will chime in!

 
 

1. Over engineering. Your headphones may be adequately driven by your headphone out but maybe you just can't be sure. A suitably chosen amplifier will guarantee that the headphones are properly driven. Unfortunately you may just have to buy an amplifier to find out.

So there are a few points to consider. Headphone amplifiers are about providing adequate voltage and current, with the cleanest possible signal, to your headphones. Unfortunately, there's a grey area where it's not obvious whether one is required; additionally, power specifications don't tell the whole story (e.g. slew rates, frequency response, available gain). Then there's the even greyer area of the subjective value of any changes...

Good luck!