Other than the answers already given,which are all right I think,its main function is to switch between being able to hear distant objects better and bring them nearer,via the top half of the knob,and to differentiate between various
different sounds at closer ranges,bottom half.For example,if pointing the HPs towards a large field of noise like a convoy,and you drop the ambient volume you can actually pick out different sounds better,like a tanker amongst merchants.
I think thats its main function,at least I use them for that purpose
Oh,and in conjunction with some good speakers and stereo headphones (surround sound ideally) and you turn your volume up even higher then you have a very good set of virtual HPs
