Why do doctors prescribe finasteride after a hair transplant?

Why do MDs suggest taking finasteride post transplant to keep the hairs? Just for keeping the non-transplanted hairs from falling out and having lower density? Nothing to do with the new hairs, right?

Finasteride after a hair transplant prevents the miniaturized hairs from falling out permanently. This is particularly important in men under 30 when their hair loss is most active.The new hairs in the recipient area do not need the finasteride to grow, just a good surgical team.