It's not like you have to wait till the last second to decide that natural birth will be too risky. With regular sonograms it's entirely possible to note when a baby is in a risky position (feet down, umbilical wrapped around the neck for example) and decide to schedule the c-section. If the baby moves into a proper position then the c-section can be cancelled, but if not they can avoid a lot of complications. Also, there are likely lower error rates from having a well planned and orderly procedure instead of emergency ones. The mother and baby could be in much danger if she needs an emergency procedure right at shift change.