Well, for one thing, they can say they offer service to destinations that they don't fly their equipment to. As a prime example, Air Canada doesn't send their own equipment here to CVG. But they do fly here. I can book AC 5XXX CVG-ORD, and then onward on AC equipment to YYZ.

Also, often codesharing is coupled with cost and revenue sharing. So, a flight from XXX-YYY might have costs shared equally by AC and Airline YY, while AC keeps the revenue from bookings it secures, while Airline YY keeps the revenue from the bookings it gets, even though its the same aircraft.

Thirdly, it can allow carriers to serve destinations they don't think there is enough demand for on their own. So if AC thinks it can fill part of a plane from XXX-YYY, but not the whole thing, they can codeshare with another carrier, who also thinks it can fill half a plane, but not the whole thing. So, instead of having to have customers connect on another carrier, AC or Airline Y it is codesharing with can both make money serving a destination they never could on their own.

Sometimes it results in competition and lower fares. Often, for example, AC and UA offer different prices for the exact same flight going ORD-YYZ. Book on the lower priced site, and maybe you can save a few bucks.

It also says on the ticket who's operating the flight (although, i agree, depending on who issued the ticket, this can be more challenging). But it keeps us on our toes, to look who is operating the flight.