The issue in a commercial kitchen is storage of raw meats in a walk-in cooler or refrigerator. If juices from raw meats such as chicken or lamb drip onto surfaces or food stored below (other meats, vegetables, etc.) they can cross contaminate those items. So, if the chicken has salmonella the drippings from that meat contaminate everything with the bacteria. This is only one of the regulations that is really strict and is a critical violation is not up to code.
What people do in their own kitchens is up to them. Also, freezer storage issues are a different matter since there is no drip factor. There the issue is more one of proper temperature for the items in question. Items may need to be separated for proper storage (ice cream VS meats for example).
