For me Kotlin is about being comfortable.
I’ve never seen another programming language (and I did production quality code in about 20 different languages) which provides as much comfort, while being a general purpose language, as Kotlin.
It is my personal taste and area of work, but for me Kotlin is a kind of perfect.
I program mostly full-stack web applications (when I have to go lower I use C++, but that’s because of speed and embedded systems).
For full stack, Java simply cannot beat Kotlin because - as far as I know - there is no Java/JS.
On the server side I could go Java, but why would I? So, I think the right question is not why I use Kotlin instead of Java. But why would I use Java instead of Kotlin?
As I see the Jetbrains team put an immense amount of work into making the language comfortable for the programmers. Not to say that they are without error, there are a few annoying things. 
Also, I cannot stress enough how powerful the new K2 compiler with the IR plugins is. You maybe don’t see it yet, but these two are amazing. Actually, for me this is a dream coming true. I can write the compiler plugins that let me write programs the way I always wanted. No more annotation hell, no more boilerplate.
My only real problem with Kotlin is the lack of identity and the lack of resource visibility. It is pretty hard to find pure Kotlin libraries. Not because they don’t exist, but because you don’t know where to start.
I think it was a good idea to build on the Java compatibility, but now we should forget Java and stand on our own legs.
Kotlin is amazing. It is not a sidekick of Java any more. It should start to act as a grown up.