I think the deal is:
[Disclaimer: I'm not a doctor, so just like anyone on the internet, I may be talking about things I don't fully understand.]
Progesterone is a hormone that your body produces naturally. After ovulation, your body should create a surge of progesterone, which nourishes the pregnancy. If you're not pregnant, there's then a dip in progesterone, which induces your period. If you are pregnant, then your progesterone levels should stay high. If you are pregnant, but you don't produce enough progesterone, you're likely to miscarry before you even knew you were pregnant.
As I understand it, SotFB is taking progesterone to increase her luteal phase (her post-ovulation period) so that any fertilized eggs can stick. I take progesterone (probably in a different dosage) to fool my body into thinking it has ovulated so my period will start. No telling when the last time I actually ovulated was.