Most US employers subcontract payroll and associated tax responsibilities to intermediary payers such as payroll service suppliers. If the company that is being outsourced is trustworthy they can help employers tremendously by collecting and consigning payroll taxes (on behalf of the employer) in a judicious manner.
Most of these third-party payroll companies generally provide good and on time service however, lately there has been a number of these payroll companies that have been prosecuted for pilfering the funds planned to pay payroll taxes.
If you are concerned about protecting you and your business against crimes like this you may visit the IRS website for more information. You can also see a list of successfully prosecuted payroll companies here.