Why do airlines change flight numbers?

We’ve all had the experience of being ticketed on, say, flight 129 and then being notified that the flight is now known as 256. There are a number of reasons this may happen. Sometimes the change is a result of a merger or a new codeshare arrangement. Sometimes it is to prevent two airlines flying flights with the same number in the same area at the same time. There is also unconfirmed speculation that the airlines may change the number on a flight that has a history of delays so that when potential travelers look for on-time statistics the unattractive history of delays will not be attached to the new flight number.