What are the advantages of a bi-directional port over a uni-directional port?

You can use the two-way communication to inform the control panel which state the projector is in e.g. ON/ OFF, Cool down/ Warm up.

Furthermore, you can make the control system act on external changes. For example you can make the status indicators (LEDs) follow the status of the projector. In other words, the control system and projector are in sync.