It keeps you on Google's site, and thus provides more exposure to their ads. Let's take the example Google give, a search for 'Matt Groening'. Without this, you'll need to click through the first result (a Wikipedia entry) to find out his date of birth. But with the Knowledge Graph that's right on the results page. You don't need to leave Google at all.
Sorry, but how does a single search for Matt Groening keep you on Google more than a search for Matt Groening followed by a click on Wikipedia?
Also, I think these types of queries (people, places, things) rarely trigger ads. Based on the example queries from the post (Taj Mahal, Marie Curie, Matt Groening) there are no ads at all.
