A straightforward answer would be air ticketing. It's well known they identify sessions and manipulate your options to give you a bad deal and them a good deal. If there's an asymmetry of info that is being exploited then this is useful.
However, I don't think anyone does this today. There are too many online ticket agent (including ITA), so it becomes obvious when the lowest fare for a given (city-pair, flight number, class of service, airline) tuple isn't being offered.