If ground beef is used to prepare a casserole, it must be cooked to at least 155F to prevent cross-contamination. This is because the casserole would be susceptible to pathogenic organisms like e-coli. These pathogens cannot be tasted, smelled, or seen but harm the consumer. Food-borne illnesses can be prevented by adopting safe cooking measures which include adopting the minimum internal cooking temperatures (depending on the food item) and prior proper cleaning of your food items.
This required minimum temperature varies depending on the food item and ensures that the germs that may be in that particular food item are killed.