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waleed:
Consider one coil of your motor only. In the motor this coil consists of both inductance and internal resistance. One equivalent circuit for this is a series inductor and resistor. Also note that the time constant of the series LR circuit diminishes as the ratio of resistance to inductance increases. So if you add external series resistance, then the time constant becomes less.
If the stepper motor rotor is not moving, then the steady state current is determined by the applied voltage and the internal coil resistance. If you raise the applied voltage the current will be increased and you will overheat the coil. Thus, you add external series resistance as you raise the voltage so as to limit the steady state non-rotation current to its rated value. By adding the external resistance you shorten the time constant and the current rises more rapidly.
Using a chopper circuit you can eliminate the external series resistance, work at a higher source voltage, maintain current limiting, and eliminate most of the external resistor's power loss.
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