If you know exactly when and where to fly, Kayak is great. But with any uncertainty, Google Flights is better for exploring. Maybe you know when to fly, but not when you want to come back. Or maybe you don't even know where you want to fly, in which case you can get destination suggestions. It's also a lot snappier.
Now with this flight data they have, I'd imagine being able to just use Google Maps route search to go from A to B such that it would also include flights where applicable. While it kind of does, I am still able to think better routes myself. For example if I search from Tokushima, Japan -> San Francisco, USA, it suggests I should fly from Tokushima. Where actually it's a lot cheaper with almost no time difference to go by bus to Osaka first instead.