What you said in the post above is wrong. Using ohm's law V=IR, as the voltage increases, the current will increase as well (assuming the device is ohmic). So for 1.7A and 3V the motor has R=1.76 ohms, and at 24V the current through the motor would be 24/1.76 = 13.63 Amps. However, this is not what happens because the motor current and voltage are controlled non-linearly by the stepper drivers.
The wikipedia link below gives some explanation of why the high voltages are needed. Using high voltages increases the speed of the motor by allowing the poles to be switched more quickly. This is done by first applying a high voltage e.g. 24V, monitoring the current applied to the windings, and then limiting that current to 1.7A (or whatever the spec is) using electronics which monitor the current.
Basically, high voltage is needed to enable an electronics trick which allows faster control of the motor.http://en.wikipedia.org/wiki/Stepper_mo ... e_circuits