Road bike pedals connect the rider to the bike via a cleat on the bottom of the road bike shoes. Usually riders will buy a clipless pedal, cleat and shoe system in one go, however on rare occasions when any one of those breaks (usually the cleat gets worn) it is necessary to buy the same brand to make sure you get a perfect fit. Pedals are a generic part of any bike and so the sizing is adjustable via the pedal mechanism. The main reason for the different in pedals is weight and so carbon and titanium combos are popular with pro riders or those looking to save small amounts of weight.