I don't know why, but I've always used Flights to book flights on UA for example.
And I've always saved hundreds of dollars. From Europe to the USA with Flights there are great rates by booking with UA. Rates that I can't find on the UA or OTA website.
The best thing about Flights is that it allows you to book some flights which would be a lot more expensive. The best times for me would be the most expensive, Flights on the other hand "equalizes" them with all the others