Mostly integration, for example some library can only be configured via env variables, but a developer might want to configure it from with-in the app it's integrated into and used from.
Also, few weeks ago I found a use for them when trying to pass configuration from Java/Kotlin to C++ library to be used during static constructors (invoked during dlopen) on Android, because at that phase native code cannot call back to JVM.
Also, few weeks ago I found a use for them when trying to pass configuration from Java/Kotlin to C++ library to be used during static constructors (invoked during dlopen) on Android, because at that phase native code cannot call back to JVM.