Let's say you want to get from the West Coast to Europe on a business class flight, but want to save some money.
Realistically, it's cheap and easy to get from West Coast airport to another, and similarly cheap and easy to get form one European airport to another.
Google flights will let you search for the best combination of flights that depart from any combination of up to 6 airports, and arrive at up to 6 airports. The technology they purchased (ITA) will let you do this as well, but limits you to a single country. Google flights? No problem with destination airports in multiple countries.
So, you could search for a flight that originates in some combination of, say Seattle, Portland, San Francisco, Los Angeles and Vancouver BC, and lands in some combination of London, Amsterdam, Paris, Madrid, Milan or Frankfurt. (Or any other 6 large European airports).
Google Flights will also show fares for a specific trip length (say, 14-days) over an entire month.
Want to filter by a maximum flight duration? No problem. Number of connections? Done. Specific airline or alliance? Easy.
In a traditional search engine, I'd have to run close to 400 individual searches to get the data that Google Flights gives me in a single screen.
It works well for domestic flights as well. I recently helped someone who had to attend a wedding across the country find a flight that was less than $180/person, when they thought they were going to have to pay over $800/person. Just by using Google Flight's tools for about 10 minutes.
There's plenty to complain about in the Google ecosystem, but Google Flights is amazing.