Two, when your child interacts with your parents it is hard not to see a glimpse of your own childhood, the part that you can't remember but can now see replayed in person. It is kind of like a little time machine that takes you back to your own youth.
And three, I am the youngest in my family. My mother was 40 years old when she had me and because of this I have grown up with the worry that my children wouldn't know her the way that I knew her. I was worried that she would be gone before they could have an interaction such as this one. Of course Esme probably won't remember this little story time moment when she is 30 but she will be able to watch it and she will always know that her Nana loves her.