Why is an antenna tuner (external or internal) frequently used with modern solid-state transceivers?

Modern solid-state transceivers are equipped with automatic protection circuits that reduce output power (foldback) when they detect a high Standing Wave Ratio (SWR). This protects the delicate final amplifier transistors from being damaged by reflected energy. By using an antenna tuner between the transceiver and the transmission line, the impedance presented to the radio is transformed to a perfect 50 ohms. This prevents the protection circuitry from activating, allowing the transceiver to deliver its full rated power output. Note that the tuner does not reduce the actual SWR or losses on the transmission line itself; it only provides a matched load for the transmitter.