As a lead in, I use a 16 ft vertical on 20-10 meters, mounted on a flat
metal roof. The antenna is fed with about 25 feet of RG-8, and there is a
tuner at the transmit end.

While I'm pretty happy with the antenna, I'd like to simplify the matching.

Thus, the question: what is the purpose of a 1:4 unun on a 43 foot vertical?
( I assume the "4" side is on the antenna side.)

I'd expect a better coax to antenna match when the antenna feedpoint is a
high Z (example, at 30 meters), but I'd also expect a worse coax to antenna
match when the feedpoint is a low Z (example, at 10 meters).

Is that the way it works, or is there other magic involved?