kotlin 委托有什么用

I'm really confused about the kotlin delegation. Let me describe the regular polymorphism approach here which looks same like the kotlin delgation.

interface Base {
    fun print()
}
class BaseImpl(val x: Int) : Base {
    override fun print() { print(x) }
}
fun main(args: Array<String>) {
    val b : Base = BaseImpl(10)
    b.print() // prints 10
}

I can pass any implemented class of Base interface to b variable to call the method of specified class's object. Then what is the benefit of kotlin's delegation? Which is described here.

interface Base {
    fun print()
}
class BaseImpl(val x: Int) : Base {
    override fun print() { print(x) }
}
class Derived(b: Base) : Base by b // why extra line of code? 
                                   // if the above example works fine without it.
fun main(args: Array<String>) {
    val b = BaseImpl(10)
    Derived(b).print() // prints 10
}

I know this is the simple scenario where the both codes are working fine. There should be a benefit of delegation that's why kotlin introduced it. What is the difference? and how kotlin delegation can be useful? Please give me a working example to compare with polymorphism approach.