Originally Posted by
catgita
One oddity of seat tube angles is making bikes for tall riders with 72.5 or 72 degree seat tube angles, and for short riders with 74 or 75 degrees. This is not because tall or short riders have different femur proportions, it is because, traditionally, everyone uses crank lengths in a very narrow range of lengths. So to maintain the KOPS (Or there about), a tall rider needs to push the saddle further back, and a shorter rider forward, following the disproportionate crank length.
That's a possible minor contributing factor, but the more obvious objective is to enable the use of longer top tubes for riders with longer torsos and shorter top tubes for riders with shorter torsos while achieving the target headtube angle and wheelbase length.