Most 6 volt dynamos can deliver 12 volts, just by using a 12 volt relay.

The perceived benefit is, that it is easier to obtain bulbs, and conduction losses are smaller.

The disadvantages are, that you have to get higher speed before charging voltages is achieved, and there is approx. 4 times as much power (heat) transmitted to the field coil.

Experiments with a Lucas dynamo, coupled to provide maximum, showed 7,5 volt at 1400 rpm (armature revs).

Current, measured in the field coil, was 2,5 amp. Effect transmitted to the coil: 18 watt 

At 2200 (armature) rpm, the voltage had reached 14 volts.

Current in the field coil measured 5 amp.

Now the effect, transmitted to the coil was 70 watt.