Benefits for the older siblings:
They will still know what to expect. They will still have the security of their routine, and the new baby will not be a cause for any major change that might make them feel uncertain and confused about their world. While caring for a newborn may shake up mom's and dad's world for a while, the other children in the house will carry on without a care in the world as they always have. We have been so thankful that the 3 times I have brought home a newborn to an older child - we have had no behavioral problems arise, no re-training or re-learning of house rules, and no aggression/jealousy/unease about the baby. Who would want to deal with all of that when you have a newborn? The transition with the new baby can be smooth, and a lot of that is a result from having a steady structure already established in your home.
Benefits for the new sibling
When a newborn is brought into the world there is a lot they must learn. They are learning to nurse. They are learning that they are no longer in the womb. They are learning who mommy is. All of these things are easier to learn if they are born into a home that already has stability and structure. Everyone is calm and understands what needs to happen, so helping a newborn to figure out their new world is much easier to do.
They will also naturally fall into a nice routine. If mom is helping to keep older siblings work within the bounds of their day, the baby will also wake and lull into a routine in which they feel comfortable. If the older kiddos always wake at a certain time and mom has to be up with them, baby should wake too and begin their day. Immediately they are starting to understand routine because they have began on one. New babies are difficult because they need so much care and attention, and it is helpful for them to be welcomed in a world that makes sense early on.
Benefits for parents
If older children are not acting out in any way due to bringing home a new baby then parents have one less "thing" to tend to. A newborn baby will always bring enough to tend to, so not having to also help an older child will allow for the transition to be pleasant, and an experience you can really treasure.
If parents have already set a sound day for older kids - they (parents) will also know when they can expect some down time or when the child will want to be fed. They can plan ahead things like trying to get a quick nap themselves or when they can get another to do list item marked off the list. They can know when they can get out and about or a play date or to run errands. They can also be sure that their child will nap. It can be planned that everyone naps at the same time, even. Most importantly, it allows for spouses to know when they can make time for each other - even in the event of a newborn baby being brought home. It has a made a world of difference to have the peace of mind that everyone in the house is getting what they need out of their day. I can honestly say that having a routine in place is the only reason I have been able to manage 4 children living under my care.
I feel like we all crave structure and boundaries. Everyone wants to live a life that makes sense. It can be so much more than surviving, and with a routine and steady structure you can enjoy each day as it comes.
Go and visit these ladies sites to see what they have to add siblings dynamics
http://www.faithfullyinfertile.com/
http://www.journeyofparenthood.com/
http://teamcartwright.blogspot.com/
http://www.wileyadventures.com/