As FWAAA said, it is mainly there to decieve the non-freq traveler.

Also, there is a slight advantage if the equipment is the same (you can't miss the connection).... The problem is, you have no way of knowing whether the equipment is the same until the day of the flight. .... I routinely have had to switch planes on the MCI-BOS (via ORD) flight.

There is sometimes also an advantage (and disadvantages), with respect to upgrades clearing and the number of stickers used.