Now that we know that Kotlin apps run on the JVM, we also know that it basically means that Kotlin can be used for anything that Java could be used for.

Today, Java is really popular for backend (server) development and Kotlin can be used for any kind of server development. Whether it is web apps that serve HTML content, a backend for REST APIs or Microservices, Kotlin can do the job. Not only that, thanks to the Kotlin JavaScript compiler, it can also be used for building frontend web Apps. So it is entirely possible to create a web App (both backend and frontend) using only one language.

Another area where Java used to be dominant was in native Android development (other JVM languages could also be used, but none has really gained popularity on Android). This is where Kotlin has probably gained the most popularity. In one part, due to Google officially supporting it for Android development, and, secondly, because Android is still stuck on Java 6. Kotlin, with its modern features, spread rapidly among the Android developers' community.

Kotlin can also be used for developing desktop apps since both JavaFX and Swing UI libraries can be called from Kotlin.