Why it's important to keep rocks clean from chalk!

For me it's a very important topic to keep the rocks clean from chalk! Using a climbing brush when you have finished isn't hard at all. It should be as self evident as it's using a climbing chalk bag. But it doesn't seem to be that self evident to others as it is to me. Therefore I want to spread my thoughts among the climbing community and hopefully everyone will brush the boulders and routes at least when he or she finished the session.