Since only two of the four pairs are used for data transmission in network cables, what is the purpose of unused pairs?

PoE or Voice

I agree with Khaled,4,5 ,7 and8 are only used for Gigabit standard,1,2,3 and6 can be use for regular Fast ethernet standards with Poe to power802.3 af compliant devices.. So with those4 pairs of cable, you can connect two devices poe or non poe and will still get speed upto100 Mb/s

Apart from PoE or Voice we can also use the unused pairs to connect up to2 pcs to the same network connection using pair matching spilter on both ends of the cable.

Both ends terminates to wall socket and distrubtion cupboard.

Can be used for POE

Twisted-pair cable is designed to take advantage of the effects of crosstalk in order to minimize noise. In twisted-pair cable, a pair of wires is used to transmit one signal.The wire pair is twisted so that each wire experiences similar crosstalk. Because a noise signal on one wire will appear identically on the other wire, this noise be easily detected and filtered at receiver.Twisting one pair of wires in a cable also helps to reduce crosstalk of data or noise signals from adjacent wires.

For the Ethernet and Fast Ethernet standards (10 and100 Mb/s respectively ) usually there is no use for pins #4,5,7, and8, but they can be used for providing Power over Ethernet (PoE). 

For Gigabit Ethernet standard (1000 Mb/s) it uses all pins for data transmission from pin-1 to pin-8 simultaneously.

PoE !

It depends on the type of use of the UTP cable, another pair can be used for PoE

crosstalk in order to minimize noise

PoE DIVICES