Employee handbooks are not legally required (unless there are state requirements or you are a motor carrier subject to Part 382 Drug & Alcohol testing requirements); however, most companies find them to be invaluable. Without a handbook, policies and procedures tend to informally change, and employees may not understand exactly what the current policy says. Having clearly written policies, centrally located, helps ensure that employees (and supervisors and managers) follow company procedures.
Plus, through the process of actually writing a handbook, you are forced to think about policies and procedures. This allows you to make enhancements and think through your policies.