I simple way to understand a chopper drive is to compare it to a car on the highway,
you could drive from a to b at a constant speed of 60, being comfortable to yourself and others, or could spurt out with bursts of excitement by zipping to 120, slowing down and again zipping ahead.
You will at the end travel the same path and would have taken lesser time.
A chopper drive for a motor is no different in concept.
It simply provides bursts of voltage much higher than that of your stepper ( mostly 10-15 times), during that time , the stepper motor can rotate much faster than what it could at its rated value.
Ofcourse , The reason we cant run it at a continuous high voltage is cause the high current will burn the motor out.
So by providing the higher voltage in pulses, it is ensured that the overall current buildup is kept at the rated current the coils can handle.