Do I need to put a HDD (hard drive) in an encoder?


It depends. If the encoder is going to be connected to an NVR/recorder, then no - the recorder will be storing the encoder camera's footage (assuming that you have a schedule set on the encoder cameras on the NVR/recorder and 'enable schedule' is checked for all of the cameras).

If the encoder will be standalone (not connected to a recorder), then yes, you need a HDD (hard drive) in the encoder so that the recordings for its cameras can be stored.