what an employee promise not to do when he accepts worker's compensation benefits

This is a complex questions. Workers' compensation payments are seldom life-time benefits, they normally are for a fixed period of time. Workers' compensation benefits are not taxed. You can file for social security benefits and medicare while you are receiving workers' compensation. Social security may claim an offset (reduction in benefits) for the amount you receive from workers' compensation. The amount paid by social security is taxed.

what are examples of benefits the united nation offers

no. If your on workers comp. then your still employeed.

so that the employee doesn't have to sue the employer for medical benefits