Technically what does "Checking In" really do?
 

It seems so basic. To "check in" for a flight. But what does this actually do on the American side? By initiating this action, does this limit any other types of actions? For instance, I want to standby for an earlier flight. I requested the standby but do I still "check in" for the original flight? By initiating check in what does this guarantee?

It also changes the e-ticket status to 'CHECKED IN'. This means e-ticket cannot be exchanged (reissued) for a new itinerary without changing the status back back to 'OK FOR USE'.

The only real reason you need to check in is to get a boarding pass, if you don't want to stop at the kiosk at the airport.

It doesn't foreclose anything else.

Well, I guess that just before they close the door, they might call your name in the boarding area if you show as checked in.

It may not "foreclose" anything else, but if you're not checked in 45 minutes prior to boarding (or 60 minutes for international flights), you may be foreclosed from getting on the flight.

Generally speaking it's a good idea to check in online if you think there's any chance you might run late (but still make it to the gate in time to board).

I can't think of any disadvantage to checking in. You can still do things like same-day change and seat change after you have checked in.

On the other hand, if you do NOT check in by the appropriate check-in deadline, you will not be able to board. Your seat will be given to a standby passenger.

For a standard domestic flight, check-in by T-45 as required assures AA that a passenger is likely to show up at the gate no later than T-15 when the passenger is subject to off-loading if he does not show.

There is no passenger downside to checking in as soon after T-24 as possible. The fact the changes may require a reissue is of no passenger consequence.

Interesting. I thought after you check in that same day change is not longer available online, and have to call in to uncheck in before you could change flights?

Someone can correct me please, but I was under the assumption that when the airline has to offload a passenger, check in time is used to determine who will be selected.

It depends on the policy set out by each airline. But yes some, if not all airlines, do use time of check in when determining which passenger to offload in an oversell situation.

Although I think there are some other considerations. For example a family traveling together where they won't off load a parent. Its probably why sole travelers get the toss (as in the Doctor on UA Express.)

I seem to recall that when travelling on a Gov ticket, up to the point I checked in, CWTSato could cancel/change/refund etc... but after that I'd have to work through the airline unless I un-checked in. Minor point, but makes a difference if my operations change.

US law requires that each carrier publish its IDB policy. Check-in time is certainly a valid choice, but class of service, elite status, fare basis and time of booking, are also factors. Often, status is the first criterion.

To a certain extent, it seems outdated and with a purpose that has passed. Delta automatically checks people in ...