Why An antenna tuner is really an impedance matching network placed between the transmitter and the feed line. It transforms whatever impedance the line presents at the shack end into the 50 ohms the transmitter wants to see, so the final amplifier delivers its full rated power instead of folding back on high SWR. Maximum power transfer happens when source and load impedances are matched, and that match is what the tuner creates at the transmitter output.
Watch out The tempting answer is that it reduces SWR in the feed line, but the tuner sits on the transmitter side of the line, so the standing wave ratio between tuner and antenna is unchanged, and the feed line loss caused by that SWR stays the same too.
A tuner fools the radio, not the feed line: it matches at the shack end, SWR out on the coax is unchanged.