Fermentation is a process where you create an environment that encourages "good" bacteria to grow. These types of bacteria grow best in air tight containers-ones where undesirable types of bacteria-including mold and yeast-cannot thrive. Because successful fermentation can only happen in anaerobic conditions, a heavy enough fermenting weight can help you create an anaerobic environment, no more worry about mold, which are a great way to keep vegetables completely submerged in brine liquid and away from air.