Since the creation of soccer tactics, no position has undergone as many changes as the defender position. As the game has evolved, different strategies both on offense and defense have required different types of defenders. As strategies become popular and copied across leagues, specific types of defenders were sought after to fill roles, leading to specialization within the general position. While this is the general answer to why, below, you can find specific types of defenders and which systems they tend to be found in.
Fullbacks are players who play primarily out wide, with the intent to both defend the wide areas of the pitch and provide support for midfielders/strikers while attacking from the wider portions of the field. The exact breakdown of those two conflicting roles depends heavily on the fullback's team's strategy and players. These players are often not as large or strong as centerbacks, but possess elite levels of speed in order to get forward and back effectively.