The biggest reason devs move to Kotlin is for the null safety. The language eliminates nearly all null pointer exceptions, because nullability is enforced at the compiler level. It's also a modern language, with many functional features. The other major feature that appeals to developers is the coroutines support, which makes it easier to write concurrent code.
Anything you can do in Java, you can do in Kotlin. The "killer app" these days, however, is Android. Google has declared that all new Android libraries there will be "Kotlin first".
The Spring framework support is also excellent, and they've added a few Domain Specific Languages (DSLs) in Kotlin to simplify development there.