Why use high voltage motors?

Most of hybrid vehicles such as Toyota Prius use high voltagemotors which reach voltages beyond 500 V.Such high voltages are dangerous and require assembly frommany hundreds of batteries.In one place I`ve read that powerelectric motor draws is related to volts times amps.I whish to know why because I thought magnetic field strengthwhich is created around wire depends on current but not onvoltage.Electric motor mostly works on magnetic field strength.