The whole idea is to get the bacteria to produce acid that will inhibit mold or other bacteria from degrading the feed.  In essence, you sacrifice close to ten percent of your feed to support the anaerobic bacteria that produce the acid in return for being able to store the remainder for a long time.  Cattle don't mind the acidity, and storing feed as a wet material allows greater efficiency in harvesting it and the high moisture allows it to be easily mixed with dry or unpalatable ingredients to build a complete ration.  Upright silos, bunkers, or drive over piles; the goal for each is to compact the feed to exclude air and seal it up to prevent oxygen from entering once fermentation begins.