Privacy. It is not relevant to this example, but it would be a good choice if the programmer did not want the sub-function to be scoped outside of the current function.

Is there a good reason to create the sub-function:

val getJustIntOrRegister = {key: String -> 
        key.toIntOrNull() ?: registers.getOrDefault(key, 0)
    }

rather than:

fun getJustIntOrRegister(key: String) = key.toIntOrNull() ?: registers.getOrDefault(key, 0)