Originally Posted by
emcampbe
Sure, but that's a big if, since this would be combining two fares to get to your destination instead of one (one UA fare for the UA segment, and an AC one for the AC segment). This would almost never be cheaper. Codesharing means the fare is done as a through fare from origin to destination.
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?