Common collector circuits can be used as current amplifiers. A Darlington configuration, also known as a Darlington Pair, uses two common collector transistors for high current applications. Since the input and output voltages are nearly the same in this circuit, it is not effective as a voltage amplifier.
Another common collector configuration is as either a standalone voltage regulator or in conjunction with a Zener diode. It clips variable or unregulated voltage from the power supply to a specific level and provides constant or regulated voltage at the output. This configuration works in voltage divider circuits also.
Common collector circuits provide effective impedance matching. Impedance matching is important to reduce signal loss in the circuit and provide maximum output.