Examples of one port network
Anything that connects to your circuit by only one pair of wires might be modeled as a one-port. For example,


An antenna
A strain gauge
A termination circuit, when modeling a transmission line
etc.



  what are they used for?


They're used for whatever they're useful for. Antennas deliver energy into free space. Strain gauges change their behavior in response to mechanical factors. Terminations determine how signal energy is delivered to a load.