Speaking from experience as somebody who has done this sort of trip: Yes, it's hugely useful and there is a real business benefit.
First off, we're probably talking about a time when there were only a few direct flights a day from SFO to the relevant airport so if he didn't go that afternoon he'd have to wait until the next morning so instead of flying through the night he'd be flying through the day - flying RIGHT THEN means one less day in which your product slips its schedule.
Now recall that Apple used to have trouble getting enough media attention so they would schedule product announcements around, say, MacWorld or the Superbowl. Big events that cannot be moved. Every day the schedule slips makes it that much more likely you miss the intended product launch window. For Apple, if your product isn't ready to ship, it's not ready to show which means you've wasted a lot of money - your next suitable launch window might be months later by which time the product no longer has a compelling story to tell. Missing the intended launch date can mean the difference between a successful product and a failure.
Conference calls are expensive and tend to involve both high-level people and lower-level people. If the schedule is slipping, the chinese engineers won't necessarily tell you the truth about why it's slipping in that sort of forum - there's a face-saving issue. There are also communications difficulties when people who don't speak english well just nod and say "yes, we'll do that" without really understanding what they're agreeing to. Email can clarify and puts things in writing but the time difference means most exchanges lose at least a day.
Another factor is "the squeaky wheel gets the grease". The same factory is trying to build products for you and a variety of other companies. When some other company's product totally unrelated to yours has a crisis, they might choose to rationally pull engineering resources away from your product to serve somebody else...if you're not there to nag and check up on the schedule.
The bottom line is that if you don't have at least one person there most of the time, your product stands a very very low chance of meeting your schedule or your quality targets.