Holy sh*t, this totally blew my mind! My first x86-based PC was a Sinclair PC200, which had CGA, and I had a RGBI monitor, and while online references of that PC suggest it had a composite output, I never used it. I thus had to endure the crappy output mode, and never knew it could have been so much better (and that a lot of the games I played were optimized for that better output).

I wonder if some emulators have a filter to get something like the composite output...