About twice a year, Expedia will still have inventory in a cheaper fare class than the airline web site. At that point, it's usually worth the $5 surcharge to just buy it from Expedia. That happened on my IAD-LAX mileage run last September.

I also use Expedia when I buy tickets for my parents. They have some health issues, so I pay the $20/ticket surcharge and get them flight insurance to cover them should they have to cancel a trip due to illness.

The final time when I've used Expedia is when I've bought end-on-end tickets and wanted them on the same locator. For example, at one point a few years ago ORD-ALB midweek was skyhigh, but competition with Southwest on CHI-BWI and BWI-ALB meant that a AA/US interline ticket ORD-BWI-ALB was dirt cheap. It was worth paying the surcharge to have it all on one ticket in case there were irregular operations.

I typically use Expedia in these situations - I like their user interface the best, and they do a decent job of finding odd routings.