Yeah it's mainly used for app development, though I've never done real mobile app development work myself.

Kotlin was billed as the drop-in successor to Java. It never caught on there, but Android adopted it, so lots of Android apps use it now.

Most interesting to me, it can compile into JS as well as into mobile binaries. So you can use Kotlin to write one app and run it on iOS, Android, and the web.

My company used it to build a cross-platform library to unify our analytics workflow across mobile and web.