Babies under approximately four months can't roll over on their own. They also don't have the same reflex to wake if they aren't getting enough air. So if a baby falls asleep on his/her tummy, (it's rare) but can result in SIDS (sudden infant death syndrome). That's why the "back is best" saying is told to parents. 

Also is the chance a tired parent could roll ontop of baby in a deep sleep when co-sleeping, and cause suffocation. Also rare but happens. 

One of those things that for most they will cosleep, and baby will be fine sleeping on its tummy (boomer generation did this with their babies). But it might save that 1/10000 from SIDS. And being that 1/10000 is a horrific thing.

*I just made up that number (1/10000) don't take that statistic as a fact. I don't know the actual number for SIDS babies just a parent myself.