Probably just that it helps their overall reproductive success/survival/fitness.
Maybe its just me as a physicist, but minimizing drag force that is proportional to the cross sectional area helps lead to a streamline body-shape. Interestingly enough mammal tails go up and down and fish go side to side so while they both move forward, the mammal tail is necessarily constrained by their land based ancestors’ vertebrae.
Gotcha, thanks for clarifying! Some placenta would still exist as even with marsupials they have a stage where they nourish their little one via similar mechanisms to the placenta-that phase just doesn’t last for as long. So less time being pregnant before a much safer birthing process.
I’m not so sure the placenta plays too much of a role in childbirth deaths (edit: I’m quite wrong on that one, thanks to @Christy for pointing that out below) but more so bipedalism and increased brain size seems to provide some trade offs.
I wouldn’t say there’s a teleology or purpose necessarily built in to the theory of evolution as its neutral to such questions. Questions of meaning and purpose are great questions, but the idea that the evolutionary process just wants to “pass on genes” is to use a language that is foreign to what one could conclude using scientific methods alone. Naturally though, offspring that live to reproduce do help pass on genes + possible variants that are slightly different from their parents.