On slotted and/or drilled rotors:

The slots or holes are there to get rid of gas that is released by the pad which can form a "bubble" between the pad and rotor which causes brake fade.

Generally speaking, the only kind of pads which have this problem are asbestos pads. Pretty much any modern pad isn't going to warrant the holes or slots. They do look kind of cool, but for the most part I'd save my money and get plain rotors.