The different configurations give different levels of flexibility in operations but basically greater flexibility involves higher cost. Therefore if the substation is in a strategic (central) location on the network then it is worth paying the extra to get additional flexibility, if the site has fewer circuits and the position is less important then there is no need for the additional flexibility and costs can be reduced. Operations staff are always looking to cover the next fault (whatever that is), even during outages. Think about when each busbar is on outage and what happens if each of the remaining busbars fault. What is the impact? How is the substation left split? Is it a minor loss of load or severance of a major flow route? What is the impact on the rest of the system?