Why do nurses push on your stomach after giving birth?

Placenta delivery after a vaginal birth
They aren't usually as strong as labor contractions. However, some doctors may ask you to continue to push, or they may press on your stomach as a means to advance the placenta forward. Usually, placenta delivery is quick, within about 5 minutes after having your baby.
If you deliver via cesarean delivery (also known as C-section), your doctor will physically remove the placenta from your uterus before closing up the incision in the uterus and stomach.

After delivery, your doctor will likely massage the top of your uterus (known as the fundus) to encourage it to contract and start to shrink. If a uterus cannot contract and become firmer, a doctor may give you medication, such as Pitocin, to make the uterus contract.
All the pushing and massaging right after giving birth is def for the placenta (and to make sure EVERYTHING is out- like pp said- you don't want a little piece of it left inside). After that they said it also helps to get your uterus back to size.