Why do green cheeked parrots bang their beak on something?

They do that because it is a natural instinct. Almost all birds at one time in their life they hit their beak on something.

Because of the nature of a parrot's diet (seeds, fruits and nectar from blossoms), seeds and sticky substances will adhere to the beak. Banging the beak is sometimes the only way these foreign materials can be dislodged.