The point of a plug is to act as a physical barrier to a subsequent male’s semen. If semen coagulates at the entrance to the uterus, it literally blocks anything from swimming through.

So thrusting (with an optimally designed penus) creates a vacuum that sucks a “soft plug” out of the way.

For species that produce a “hard plug,” the bone is useful to dislodge the hard plug. In laboratory rats and mice, the hard semen plug is visible, and it is actually used by scientists to determine whether animals successfully copulated.

It is well established that species with more prominent semen coagulation exhibit more female promiscuity, which is consistent with the notion that a man must compete by forming semen plugs, and by removing semen plugs from other males.

But seriously, there’s also the role of female orgasm and timing thereof during intercourse that allows a woman to chose which parner is more likely to father her children.