Of course, the new air travel bill will prevent airlines from deboarding people who have boarded when the flight is oversold.

I also remember when it was not possible to change seats online after checking in. I now see this is possible.

IME airlines rarely ever bump someone already on board. I've had an announcement made asking for volunteers during boarding. A couple years ago I was sitting in 1A and the GA came on after boarding and asked for a volunteer ($500 voucher). I jumped up and volunteered. He told me to gather my belongings and he'd meet me at the podium. After about 15-20 minutes he finally comes back to the podium and tells me to go ahead and reboard.

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So if you check in for a flight from xxx-yyy, and that flight is delayed for 3 hours, and there are empty seats on the next flight from xxx-yyy that will now leave before the flight you checked in for, you're not at a disadvantage of getting rebooked on the later (now earlier) flight compared to someone who hasn't checked in? All other things (status, fare paid etc) being equal? It seems like it'd be easier/path of least resistance to put a non checked-in passenger on a different flight than one who has checked in. In any event I feel fairly sure there have been at least a couple of instances where I felt fortunate to not have checked in early, that my situation was better than if I had checked in, though I may be mistaken. I think the overwhelming majority of times it is beneficial to check in as soon as you can, but "no passenger downside" at all? Ever?

I thought checking in was a more formal - and since 2001, perhaps a more regulated - procedure that replaced the old reconfirmation (call the airline 24 to 48 hours before departure, reconfirming your reservation). Back then tickets were essentially transferrable coupons, so passenger names and individual data were less important than head count.

But Dr Dao was travelling with his wife, (who supposedly agreed to deplane)
https://en.wikipedia.org/wiki/United..._3411_incident

I think this is it. It also serves as a reminder for people the day before the flight to ensure that everything is good relating to their travel plans.

Thats my point.

No PSS (passenger service system) I have ever used at an airline would require extra or more complicated steps to move a check in versus non checked in passenger in the scenario you described. So no downside to being checked in.

Thank you OP for asking, as it is something that I don't really understand either. If airlines will have an option to automatically check you in, (which seems far from uncommon these days) what is the point of it in the first place? Why does it need to be a different function from purchasing the ticket/reservation? Especially considering earliest check-in times can range from 2 hours (when you have to check in at airport) to 60 days, at that point, what is the purpose of it as an additional step?

Somewhat related - Failure to check-in and pull up your mobile boarding pass prior to boarding beginning could make it impossible to get on the flight, which is relevant if you are cutting it close. https://www.washingtonpost.com/news/...=.53d10dc2aa29