This is a relatively new phenomena that has come up in the birth world as more and more research on the significance of gut health and our microbiomes has emerged. In birth, this applies in particular to a vaginal vs. Cesarean birth. During a vaginal birth, the baby is colonized, or "seeded" with bacteria from the mother's vagina, which leads to long term health benefits; in a Cesarean birth, the baby does not have the same bacterial colonization opportunity. It is because of this that some women are looking to simulate this process at the time of a Cesarean.
We are just now beginning to understand that many modern day health conditions, such as colon cancer, Autism, and mental health (just to name a few) are likely affected by the balance, or lack of balance, in good and bad bacteria in our bodies. Think of the recent surge in the understanding and promotion of probiotics, our understanding of antibiotics, and the fermented food movement.
The recent documentary "Microbirth" focuses on bacterial health in birth, as well as some recent articles below. You can also check them out to understand how some women are choosing to take vaginal seeding into their own hands in the event of a Cesarean.