A recent flight that I was on was far from full and not a long one. Once we were at cruising altitude I moved to the unoccupied right rear row. A FA in the last row on the left chased me right out of there. No explanation just “You can’t sit there!”. O.K. Jeez. I wasn’t smoking or anything.
Once the gate has control over the flight or in the case of irregular operations, they can assign any seat.
I just read a thread on travel stack exchange, on the same topic, where there were two answers: one was something about center of gravity, which seems like it would be less of a concern on an A330, but what do I know and the other was
In long flights, in airplanes that has no crew rest area (bunks) such as A330, airlines reserve the seats at the rear of the airplane for the crew to take rests (shifts).
So that might be it, though I don’t know if a little over 7 hours counts as a long flight.
Which backs up @suranyi’s response above.
We are getting firmly into country specific and airline specific territory there. @Maserschmidt was flying to Germany with Lufthansa and my experience is the other side of the world which may be different to the US.
My old airline didn’t sell stand-by tickets to the public. The people waiting for last minute seats were either employees or their friends / family on a staff travel ticket, or they may be travelling public who have requested an earlier flight and it’s been granted on a standby basis.
I have never NOT got a seat when travelling standby but I have had to wait until the last minute, literally, and I have also had to take a jumpseat in the flightdeck.
If you don’t have luggage and have checked in already, you can turn up to the gate right up until they close boarding. The standby travellers will be hanging around anxiously waiting if there is a seat for them and their names won’t get called until the gate closes. It is often the case though, that the ground staff can see there will be enough empty seats and will start calling names earlier.
As for rewards seats and so on, I don’t think that makes a difference, a seat is a seat. They’re not likely to give first class seats away to standby travellers, but there is absolutely no reason why they wouldn’t give a standard economy seat that has been vacated by a super full flexi ticket holder to a standby ticket holder. Indee the full fare paying passengers are the most likely to not show up, because they are the ones who have the flexibility to change their ticket at no cost. Rewards are a liability for the company, it is in their interest to have people use them.
Edit: I should point out that there have been times where I wouldn’t have got a seat on a standby ticket but I realised this in advance and changed my ticket to a firm seat on a different flight. So there’s some self selection bias going on there.
American Airlines always warns that if you are not at the gate within ten minutes of scheduled departure time you may lose your seat.
" You moved the factory but you left the bodies behind !!! 