Originally Posted by
zkzkz
This is the real answer to the original question. All the other answers about what it allows the airline to do that they couldn't do otherwise are clearly bogus rationalizations.
I'm surprised the booking sites don't hide this from the passenger though. I would be a lot happier if I used expedia or kayak and it just showed me the logos and flight numbers of the actual operating company. Why do I need to know who's handling the money?
Probably too complicated from a technical/back end point of view, and how they need to handle changes/cancellations if applicable, and probably much easier to align their internal accounting systems with your booking.
If you book yourself on an UA-operated, AC-codeshare flight, for example, AC still handles changes, etc. and that's who's fees get assessed for changes, etc. Orbitz can't go to UA to try and change your ticket. Just like if you booked it on AC.com, UA would only handle changes on the day of flight at the airport. That's probably a whole other can of worms, and additional set of costs those sites would have to deal with.