What is one reason to use an impedance matching transformer at a transmitter output?
Why Modern solid state transmitters are designed to deliver full rated power into a 50 ohm resistive load, and they reduce output (or shut down) when the SWR rises. An impedance matching transformer or antenna tuner transforms whatever impedance appears at the end of the feed line into the 50 ohms the transmitter wants to see, allowing maximum power transfer. Note that it matches at the transmitter end; it does not change the SWR out on the feed line between the tuner and the antenna.
Watch out Reducing power output is what happens when the match is bad, not the purpose of the matching device, and radiation resistance is a property of the antenna itself that no transformer at the transmitter can change. Power supply ripple is a filtering issue in the DC supply, unrelated to RF impedance.
Transmitters want to see 50 ohms; the matching transformer is there to give them exactly that.