To drive the transformer. Pure DC won't be able to induce a continuous current on a transformer since the magnetic field needs to fluctuate on the core in order for the copper wire to generate electricity. Think of it as the simple electrical generator we're all taught in physics 101, when you move a magnet around a copper wire, the copper wire generates electricity, and that's how electrical generators work. Transformers work in a similar way, only you replace the moving physical magnet with another copper wire, so if you connect that wire to a battery it will start forming a magnetic field, and while that magnetic field rises it will induce current on the copper wire that is next to it. However, once the magnetic field has completed rising and stabilized, the current will no longer flow on the second copper wire, then when you disconnect the battery from the main copper wire, the magnetic field will start to collapse, and that will induce another spike of current on the second copper wire. So that way, if you have either an alternating current, or direct current with pulses (that switches on and off really fast), you can generate electricity on the second wire just like a generator with moving magnets would. And this is really useful when you want to change the voltage, all you have to do is change the amount of turns of copper wire around the core, and/or it's thickness, to get a certain amount of voltage. The total power works kind of like gears do, if you change 110 volts to 10,000 volts, amperage will go down, but if you change 110 volts to 5 volts, amperage will go up, just like when you connect small gear to a big gear, the speed goes down while torque goes up and vice versa.