Given that nothing happens until the gate, you still need to interact with an agent, and the amount is not locked in by any means, it isn't even a bid.
But, it's great data for UA. Perhaps if they see that there are a lot of people willing to move for smallish amounts, they will start offering lower VDB's and perhaps they will broadly implement their pilot program of moving people early along with a smallish voucher on flights where it's fairly certain that the overbooking will become an oversale.