Exactly. Codesharing gives the impression of a complete schedule for all participating airlines while not requiring that each airline operate the full complement of flights. AA wants to begin unlimited codesharing with jetBlue and Alaska (and perhaps some codesharing with US) so that AA won't have to duplicate the schedules flown by those airlines. But with lots of codeshares, it can tell its corporate customers "look at this comprehensive network we have in the Northeast or in Seattle/Portland." Adding flights on AA metal would drive down fares unless those flights spark a huge uptick in demand.

NRT-ICN features several widebodies every day, and it appears that there's a lot of cross-codeshares (so that each airline can offer its customers the convenience of a large number of flights without losing money by flying 5-6 flights a day).