Why An antenna tuner (more accurately an antenna coupler or matching network) is an adjustable network of inductors and capacitors that transforms whatever impedance the feed line presents into the 50 ohms the transmitter wants to see. It does not change the SWR out on the antenna itself; it just presents a friendly load at the radio end so the transmitter's protection circuits do not fold back the power. Modern solid-state finals need a close match to 50 ohms to deliver full output, which is why tuners are so common.
Watch out The choice about tuning in weak stations describes a receiver's tuning or preselector circuit, not an antenna coupler; the one about using an antenna for transmit and receive describes a T/R switch or duplexer, and automatic antenna selection is the job of an antenna switch.
A tuner tunes the transmitter's view of the load, not the antenna: it makes the rig see 50 ohms.