Much like a new set of pistons and rings in a freshly bored cylinder, these moving parts need to mate to function properly. Looking at the ground surface of a new brake pad and rotor [new or used] under magnification, the surface looks like a bunch of hills and valleys. Until these parts are mated, only the peaks are making contact. If you try to use the brakes hard before they bed-in, you run the risk of over-heating and carbonizing the surface of the pad. This means performance will suffer dramatically and a glazed surface will likely result...you'll probably hear your brakes complaining via a loud annoying brake squeal. Before installing new pads, clean the brake rotors by using medium grit sandpaper on the pad track followed by wiping them down with a clean rag and acetone or denatured alcohol. Allow some time to bed-in your new brakes by following the manufacturer recommendations and you'll find they'll work much better.