For the marketing airline it allows routes and fares to be constructed using their own revenue management systems. BA remains far bigger than AA in India so connecting India to relatively small cities in the USA, using their own ticketing, fare rules, and allocated inventory is a clear benefit. For the operating airline it allows them to shift short distance inventory - particularly middle timeframe inventory - and thus allow them to squeeze up fares closer to departure for short notice bookings. I agree the management of codeshares is problematic in terms of needing manual intervention, compounded by the fact that AA, a huge airline, does have to rejig its schedules frequently. BA's codeshares on say Loganair or Qatar rarely cause these issues, so some of this is unique to AA.