Aside from what other people have mentioned: I use ITA because it lets me see the fare rules, which helps me anticipate potentially useful mutations to my ticke

Aside from what other people have mentioned: I use ITA because it lets me see the fare rules, which helps me anticipate potentially useful mutations to my ticketing. (For example: Knowing that a particular round trip to Paris allowed for a free stopover in London allowed me to attend a conference in the UK for a much lower cost than if I had booked a round trip ticket to the UK.)