Building a Distributed Runtime for Interactive Queries: Apache Kafka and Vert.x (blog.codecentric.de)

I was just discussing the use-cases of Kafka Streams with a colleague today. Can someone explain how having only local state is advantageous for most applications? It would seem it's lacking key functionality if you still need to build a query layer that glues all the topic local states back together for a global state - like the author of the article is doing.