The directional coupler has rf-properties which make it useful for tuning circuits. It has a low-loss path between IN and OUT . If an rf-source is applied to OUT port, practically the full level will appear at IN port. If the IN port is terminated into 50 W , no power will be reflected. Thus, if a scope is connected, we can observe the reflection.
Directional couplers can be very broad band.
If in doubt, consult manufacturers specs. Rotating the plug-in selects forward or reflected power display. The biggest advantage is simplicity of operation. The drawbacks include the need for different plug-ins. Only the more elaborate models are capable of operating in pulsed mode.