According to this article, a seat tube angle between 65 and 110° has little effect on max power output. At most, it varied 1%. https://www.me.utexas.edu/~neptune/Papers/jab26%284%29.pdf
Adjusting the setback would affect the effective STA. The reason for a more forward seat position seems to be an aerodynamic position.
If it works, it can allow wrong sized bikes to fit. If the bike is too large, the reach would be too long so to decrease it, the seat is moved forward. If the bike is two sizes too large, one might flip the seat post to move the seat even further forward. The opposite is true for a bike that's too small.
Is it possible it's rarely done because it affects aesthetics, preferred postures, safety reasons, or weight distribution?