I personally think itīs issues due to flight loads. For routes like ORD-HKG nonstop, those flights are often sold out way in advance. Offering flights like ORD-SFO-HKG, which will allow airlines to spread passengers out, instead of getting them to go on a waitlist etc. Itīs just more options to give to the passenger and to keep them happy in case some flights they want are not available.