What is the benefit of having hardwired smoke detectors as opposed to battery operated?

Several actually.  First, a hardwired smoke detector has 120 volt power and a battery back up so no matter what you have a functional smoke detector, even if the power goes out.  Second, usually hardwired smoke detectors have a signal wire connected between multiple them that allows them to communicate to each other when there is smoke.  This means that even in a large home if there is a fire on the other side of the house the smoke detector in your bedroom will go off giving you plenty of time to evacuate.  Third, you will be in compliance with current code requirements.