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

An antenna tuner, more accurately called an antenna coupler or transmatch, acts as an impedance transformer. Modern transceivers are engineered to deliver their maximum power into a 50-ohm load. If your antenna's impedance is something other than 50 ohms, an impedance mismatch occurs, leading to an elevated Standing Wave Ratio (SWR). When placed between the transmitter and the feed line, the antenna tuner transforms the mismatched impedance of the antenna system into the flat 50-ohm load the radio expects to see. This prevents the radio's safety circuits from reducing output power and allows the transmitter to operate efficiently. The distractors represent other station functions. An antenna tuner does not physically alter the antenna's length or resonance, nor does it help a receiver 'tune in' weak stations like a VFO dial. It simply ensures the electrical match between the radio and the antenna system.