Kotlin, classes inside interface, interface inside classes, sealed classes, data classes and so on.

I recently started reading kotlin docs, it has a lot of features in oop. Its cool but at the same time i can't help but wonder why do we need this? I was doing fine with data class, sealed class, inner class and such but interfaces inside classes, classes inside interfaces and so on just blew my fuse.

interface AnInterfaceIsJustAContractNothingMore {
    class WaitUntilYouCodeInKotlin {
        fun OhNo(): String {
            return "I cant even tell the difference anymore. Its like blending diff realities."
        }

        interface ThisWorksToo {
            class EvenThis {
                val why = "Why do i need this?"
            }
        }
    }
}

I am writing this to express my opinion at the same time have some opinions on how other people feel about this?