Workers' Compensation covers the medical and rehabilitation costs of your employees if they're injured on the job. (It also may cover some lost wages.) Having insurance that covers these costs means employees give up their right to sue your business for negligence and in turn, they get peace of mind knowing they can recover for work-related injuries without the complexity of a lawsuit.
If you have any employees most states require that you carry Workers Compensation insurance and the fines charged if caught without it are extensive. If an employee (or even a friend or relative) are injured without being covered by Workers Compensation they can sue you and potentially take everything that you have worked so hard to protect.
In some states farming employees and self-employed persons are exempt and are not required to be covered by Workers Compensation.