One of the features I like to use a lot is the ability to specify fair classes. As an example, say I want to fly to Tokyo, and I am an Alaska Airlines mileage plan member. Alaska Airlines does not fly to Tokyo, but it has deals with airlines that do. However, sometimes the fare classes are quite complicated. For instance, "Economy" is broken down into many buckets, and not all are created equal. For instance, Delta has at least 13 buckets for Economy, and they each 'fare class' award different amounts of mileage to Alaska flyers [1]:

E: 25% Mileage

L, U, T, X, V: 50% Mileage

H, Q, K: 75%

B, M, S: 100% Mileage

Y: 125% Mileage

If you search on Google Flights, these will all be called "Economy". If you search on most of the other OTA's, you can sometimes find the fare class during checkout or even as part of your search results, but you can't filter on it (Hipmunk is one that does support some of ITA's syntax for these filters, but not all). The buckets aren't always strictly more/less expensive, but they're usually not exposed very easily, if at all[2]. So, you're often left crawling from listing to listing, expanding to see if they are going to get you any miles. (I'll save the debate of whether miles are worth all the effort for another day.)

On ITA, it's not unreasonable to construct a query that says "During the month of November, show me round trips that are between 12 and 19 days that are going from Denver to either Narita or Haneda Airports, which will earn me more than 50% miles on either Delta or American or JAL, but also only ones that connect in Portland or Los Angeles, with no prop planes or overnight stops, and no <50 minute connections or 3+ hour connections". (I wouldn't actually specify all of these stipulations, but they're good for the example! :) )

[1] https://www.alaskaair.com/content/mileage-plan/how-to-earn-m...

[2] Delta, to its credit, does allow you to search by minimum fair class on its advanced search)