Very often the pads will be somewhat torn up from the old rotor surface, especially if the rotors are scored or dished. It that case, the old pads will tend to score the fresh surface on the rotors all over again. Also sometimes the pads will be a bit glazed or may even have some metal transfer from the rotor onto the pad.
I have often reused old pads if everything looks fine and they are nearly new. If they are even half worn it is wise to install new pads so that you do not have to do it again very soon. The rule of thumb is in there is any doubt, change the pads.
