UA does this when they have a fairly massive overbooking and pretty much know that they face oversale. They know that they can get people to move for relatively little because there's no delay at the airport.
The agent discretion tends to be limited because UA knows that if it can't deal with the issue in advance, there is still check-in and the gate before an IDB problem arises. That's when the big offers come in, but it's not likely to happen because it's a reverse auction and someone else will have grabbed it.