Stepper motor windings have significant inductance, so the current takes time to rise and fall. This reduces the torque as the motor turns faster.
Applying a bigger voltage reduces the rise and fall time of the current , so the motor torque falls more slowly as the motor speeds up.
If the voltage applied is bigger than the motor rating, the motor current has to be limited . Resistors can be used. They don't have inducance, so they don't slow the rise and fall much, they just limit the maximum current. This is a very simple improvement that can be made. The downside is that the circuit is very inefficient, with most of the power going to heat the resistors.
A better but more complicated solution uses a current control circuit. In that, a large voltage is applied to the winding. When the current gets to the rated vaue, the supply is removed and the current continues to flow in a freewheel diode. The current is controlled by PWM.
When the current is to be stopped, for the next step of the motor, the freewheel diode is disconnected, forcing the coil current through other diodes, back into the supply. That reduces the current as fast as it increased.
This is much more complicated than some big resistors, but performs better, and uses much less power.