Output chokes prevent damage to the motor insulation by surge voltage. AC-motors, especially inverter driven motors should bear a du/dt of less than 2kV/µs. Other ac-motors should bear a du/dt of 1kV/µs). Output-chokes will also reduce emissions and help to meet the EMC-requirements .
Frequency inverters of type vkv... have a built-in output-choke. To operate older motors, extra chokes should be used.