Erasing your heart, emptying yourself of all worldly concerns and desires, in Taoism and many eastern religions it is the ideal state to live in. Think of it like a dream state where you are both detached and immersed in your surroundings.

Koishi basically goes around and fully submerges into the world of Gensokyo's surface every day, and her morning conversations with Satori are the affirmations of the one remaining heart connection she had after otherwise closing herself off entirely. All the things she sees happen with others she inserts herself as the main character, due to our tendency to unconsciously identify with the people we see and like.

In the end Koishi let Satori experience her world. A world where by erasing your heart you essentially become the hearts of those around you. This is mirroring Satori's experience of being an isolated ego in a hostile foreign universe of others.