Let's say you're searching Google Photos for "Mammals". Photos might have neural tags tagged for dogs, for dog breeds, and for very specific animals, but they might not be tagged for higher level concepts, like that a Dog is a Mammal.
The KG tells you if a given term is a Point of Interest, a location, a landmark, a person, an actor or character, a film, food, and what not. This lets stuff like Now on Tap provide contextually relevant apps like IMDb for films, actors, etc or Open Table for Restaurants.
I've found KG to be very useful for some queries, especially queries that produce Carousels, like "top ten movies in 1996"
From the article:
If you’re listening to a Skrillex song, you can say “What’s his real name?” and your phone will give you his real name. If you open an email asking if you want to see Tomorrowland, you can tap on it and get instant reviews, ratings, and trailers for the latest sub-par George Clooney flick. If your get a text suggesting dinner at some hip restaurant you’ve never heard of, you can tap again for reservations and directions.