There’s a moment of nervousness for many new parents as they put their baby down into bed and walk away down the hall, out of sight, for the rest of the night (theoretically, of course - you know exactly how likely your baby actually sleeping through the night is). Room-sharing, or keeping a baby in her parents' bedroom for the night (though not in the same bed), is some parents’ way of compromising with that feeling.
According to the American Association of Pediatrics, room-sharing (but not bed-sharing) in early infancy can reduce the risk of SIDS greatly. It’s hard to say exactly why this is, but it probably has to do with the fact that parents are closer at hand if babies have any problems, and can notice and respond faster if anything comes up. In fact, the AAP recommends that babies share a room with their parents for the first year of their lives, although a June 2017 study published in Pediatrics suggests that room-sharing may only be helpful for the first 4 months of a baby's life.
Recommendations aside, nursing mothers also sometimes find that sharing a room with their babies can help them get more sleep. Keeping babies nearby and in sight can also help new parents feel more secure.
The major argument against room sharing is that it trains babies into bad sleep habits which give them a harder time transitioning to sleeping in another room on their own. To take this idea further, some sleep researchers believe that any amount of parental involvement and presence when a baby is falling asleep can lead to a lighter, more broken-up sleep.
Some of the other drawbacks of room-sharing are obvious - for example, even with all of the potential benefits of room-sharing, if you smoke in your bedroom, it’s still probably better for your baby to sleep somewhere else. There’s also a definite loss of privacy involved in room-sharing in ways that bothers some new parents more than others - but there are other reasons why room-sharing might not work for your family that might catch you by surprise. For tiny people, some babies sleep really loudly. If you find that sleeping in the same room as your baby is keeping you from actually sleeping, room-sharing might not be the way to go.