What is actual advantage of backing property in Kotlin?

Question

I recently started working on Kotlin getters and setters and thereby landed on topic of Backing Properties in Kotlin. I am aware of the concept of Backing Field in Kotlin. I was refereeing to a book called Kotlin in depth by Aleksei Sedunov, in which the author says-

If you need to access a field of a property inside a class in which it's declared, you can use backing properties.

While reading the book, I came across this medium article posting the code:

class Human {
    private val _age: Int = 20
    val age: Int
        get() {
            return _age
        }

     val printAge = {
         println("Age is: $_age")
     }
}

My question is, what is actual advantage of defining a backing property where we can, rather, access the actual variable itself using this inside the class ? Like, in the printAge function, rather than defining the backing property and then using it, couldn't have we just done this.age ?