When playing the harmonica after a meal or snack small depsoits of food a left within the instrument. This is obviously a design feature to be applauded as it means that a small snack will be available for later consumption whilst cleaning the reeds of the harmonica. Now, it would simply be unhygenic to allow such tasty morsels to remain damp due to excess spit being all over them. That's why we tap our harmonicas.

It's a food health and safety issue. ;-> Paul