An oft overlooked game mechanic that allows the player to breed new life, and new rewards.
Sometimes this is a core game mechanic where one of the main aspects of gameplay is breeding. This could be to create stronger units for yourself or populate your village. Usually though, breeding is a smaller aspect that is actually found in many games. Usually in these instances, it is integrated into the player's inventory management or automated in some way so it might not be obvious to the player that he is breeding.
In both instances though breeding usually will reward the player with rare units, items, or other bonuses if they breed in a certain way. Whether it be by combining higher rated items or unusual items to get a very useful or powerful unit.