Final answer:
The package insert is required for new drugs and generic drugs to inform users about safe and effective usage.
Step-by-step explanation:
The two classes of drugs where the package insert is required are new drugs and generic drugs. The Center for Drug Evaluation and Research mandates that these drugs come with a package insert since they either contain new active ingredients or are equivalent to already marketed medications but produced by different manufacturers. A package insert ensures that the drug is used safely and effectively by providing critical information such as dosing instructions, safety warnings, and potential interactions with other drugs.