Some power user features of ITA aren't available on Google Flights still - for example, it lets you construct specific weird routings or layovers that Google will often optimize away. For instance, if you want a 24 hour layover in a city, or if you want to fly a less efficient direction to your destination to build mileage, or whatever.

You can use bookwithmatrix.com, though, which parses the output and will generate a ticket for booking on an online travel agency like Expedia.