gives u the flxibilty of running ur woofer on various kinda amps,
suppose u have dual voice coil each 4ohms,
connect them in series and u have 8 ohms load, easier for amps to run, thou less power.
conncet them in parallel and u have 2 ohms load, hard for amps to run, thou more power IF ur amp can provide it.
connect only one coil, u have 4 ohms, still easy load to drive for amps, thou u lose about 30% of ur maximum power input.