confused- progesterone after ovulation/TTC vs inducing a period

So my doctor prescribed progesterone vaginally 100mg twice a day starting at 3 DPO.  I'm confused why some take progesterone to induce a period, and why some take it to help with implantation like I am? What's the difference? I'm suppose to start mine today just want to make sure it's not going to "induce" my period while I'm TTC. Thanks!