Most VFD's don't accept a PWM signal.
The UB1 converts the PWM to a 0-10V analog signal, and requires a 10-12V DC voltage. You can usually get the 10V from the VFD. However, there's a slight voltage loss in the PWM >0-10V circuit, so to get a full 0-10V range, you need a 12V supply. If you can't get 12V from your VFD, then yes, you need the 12V supply in order to get your 0-10V speed control output.