I would say be able to book fares that are unavailable on the airline's website.

Flight Network through BookWithMatrix is excellent for booking fares that are only available on the Google ITA Matrix.

Sometimes the UA website gives me an error on the last page when I try to book a fare. Their phone agent always say it's because the fare is no longer available. However 99% of the time I'm able to book it through Orbitz.

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On a sidenote most OTAs are scammy. The #1 biggest scam is not allowing you to cancel within 24hours for free (or having to pay a protection fee for it).