Most airlines' position is that they have to oversell flights, otherwise planes will fly with empty seats unnecessarily.
But from passengers' point of view, there aren't always enough people who'll voluntarily accept an offer to transfer to a different flight once they are at the gate awaiting boarding [this sentence was edited in response to kortilla's comment below].
Volantio's product applies sophisticated analysis to each flight's booking levels well before departure time (days or weeks before), and will send offers to passengers to offer incentives to transfer to less-full flights, allowing plenty of time for them to change their plans.
So far the product seems to be having an impact for passengers [2], gate staff [3] and airline management [4].
[1] https://news.ycombinator.com/item?id=16836991
[2] https://www.independent.co.uk/travel/news-and-advice/qantas-...
[3] https://www.volantio.com/alaska-airlines-improves-denied-boa...
[4] https://www.business-standard.com/article/companies/indigo-p...
Disclosure: Volantio was a pivot from Adioso (YC W09), of which I was a co-founder. I'm no longer an active employee or significant shareholder. However I remain friends with the team and am pleased to see them doing well and solving an important problem.