Codeshares serve the pax as well -- airlines can offer direct services to other destinations that they would not otherwise serve, more complex but seamless interlining, improved marketing of services, and most importantly for the consumer, fare rules that are uniform and typical of the carrier. Otherwise, you could be subjected to different terms of carriage and fare rules as you continue to change carriers, even if it's on the same ticket.

In reality, codesharing is a glorified way of doing interlining, with benefits.