Babies under 1 year of age do need to sleep on a hard flat surface because babies under 1 year of age can roll over onto their side or stomach and bury their face in the soft materials causing them to not get enough air to breathe.
When they cannot turn back over and can't breath the baby can die as a result.
So if the baby is under 1 year of age then you should use a firm flat surface without any pillows or soft objects to prevent the death of the baby.
Once the baby gets to 1 year and above they can sleep on any surface and have soft objects in crib or bed with them.
But until then keep the baby on a flat hard surface to prevent SIDS.