The reason we cleanse crystals, besides the fact that when we bring them home they may have debris from the earth where they were mined, is because they also pick up and give off vibrations.  For this reason, you want the vibrations to be cleansed from any negativity or extraneous energy that you want to be removed from your crystals.  Crystals indiscriminately pick up energy.  Because of this, you want to keep them as clean both physically and spiritually so they can work the most effectively for your intentions.