Brake resistors are used to prevent the DC Bus voltage of the drive, exceeding a preset value during the ramp down or the decel period.
Brake resistors tend to be used when you have a fairly large load that requires a fairly fast deceleration rate, without this resistor the drive may trip on an overvoltage fault during decel, this could, dependent on the application cause a safety issue should this drive trip has you will lose control of the motor during ramp down, would like to know more about the application.