Say what you want about Google/Alphabet as a whole, but Google Flights is incredible. It's absolutely amazing how quickly and largely that Google crushed all its competitors in terms of usability. Kayak, Hipmunk, Orbitz, Skyscanner were all my go-to's at one point but G.Flights is far superior in so many ways. It actually remembers your airline preference between searches. You don't have to beg or change settings to…
Funny story: just last week I was booking a complicated multi-city trip with 5 legs, multiple countries, etc. I tried several times via united.com, and it wasn't showing me any flights available in one of the legs (which I knew they had a codeshare agreement).
I went back to Google Flights, chose all my flights, selected "Book with United", and was able to finish the reservation on united.com (!), with all the flights properly populated, including codeshares.
It's remarkable that the integration between Google Flights and the airlines is better than even the airlines' websites themselves.