The Knowledge Graph is a free database of entities and their relationships maintained by Google. This database is based on the information available from free sources like Wikipedia and Freebase. But, Google fears the information provided by these sources might be inaccurate and this is the reason Google displays the feedback option along with every Knowledge Graph in order to allow people to locate the possible errors. Another possible reason might be that Google Knowledge Graph algorithm which is still in infancy might go wrong in judging the relationships between the entities. A feedback in time will allow Google to locate the error and use it as a base for future algorithms that will be more powerful in determining relationships than the existing one.
Google wants us to provide feedback if the information returned by the Knowledge Graph is wrong. Have a look at the sample below for the query "rhinoceros":
Clearly, Google doubts either the information available from its sources or its own algorithm and requests user feedback on the accuracy of the information. Every Google user is playing a part as a Knowledge Graph tester.
Google knows that in its quest for becoming the number 1 answer engine in the world, it must have a database of real world entities and their relationships that can assist it returning accurate and faster results to the user.
Best of luck Google and the Knowledge Graph.