What is the major function of an antenna tuner (antenna coupler)?

Modern amateur radio transmitters are designed to deliver power into a 50-ohm load. If your antenna system has an impedance other than 50 ohms, this mismatch causes power to reflect back down the feed line to the radio, which can reduce output power or even damage the transmitter. An antenna tuner acts as an adjustable impedance transformer, presenting a perfect 50-ohm load to the radio while matching whatever impedance the antenna system actually has. Despite its name, an antenna tuner does not physically alter the antenna itself, nor does it help a receiver automatically tune in weak signals. It strictly matches the electrical impedance between the transceiver and the feed line to ensure safe and efficient power transfer.