Following on from what Mike says above, when you replace the rotor and keep the hat CHECK WITH THE MANUFACTURER, most recommend (require) that you use NEW bolts and nuts to retain the rotor to the hat, and they're usually specific hardware from the manufacturer. Something to do with the bolts stretching when torqued, kind of important not to overlook if you like your brakes to stay assembled at speed
The aluminium section is also better at dissipating heat than a single piece cast rotor.