What is so important about the ending scene in The Last Lecture?

The final scene of The Last Lecture is so important because it reveals Randy's real purpose.
Randy opens The Last Lecture by saying his work is "not about dying." Rather, the lecture is about how to live. Randy accentuates this with talking about his life experiences and the lessons he learned. Throughout the work, the feeling is that Randy is trying to give a primer to the audience as to how life should be lived and what should be done in it to make it more meaningful.
The final scene challenges this idea. It is so important because Randy tells us the real reason why he decided to give the talk: "The talk wasn’t just for those in the room. It was for my kids.” When Randy tells us the talk was intended for his children, we understand he truly is going to die. What we witnessed was a dying father's wish for his children because he will never get to see them grow up. The lecture is a way for Randy to transcend his own limitations and speak to his kids after he is unable to do so. When this revelation dawns on the reader in the final scene, it adds poignancy to a work that is replete with it.