Codesharing is intended to be an advantage for the airline, not for "you" the passenger. Some airlines have contractual arrangements -- e.g. company X will always buy tickets on airline A for any route served by A (in return for a substantial discount). Thus when airline A adds a codeshare to a new city, it is guaranteed this captive business -- which wouldn't be the case using a partner airline in an alliance. Some countries also require contractors to use airlines flagged with that country. Thus a US contractor must fly an UA codeshare instead of AC (or an AA codeshare, for that matter). These codeshares are easy to spot in the schedules, they are often priced much higher than the bare metal prices (or a connection to the same airline doing the actual flying). The primary purpose of codeshare vs. using an alliance is to capture this captive business, at a profit. There really is no other purpose, it is all about profit maximization.