Students do not come to school as blank slates to be filled with information. Rather, they come to school with considerable knowledge, some correct and some not. Either way, that knowledge is based on intuition, everyday experience, as well as what they have been taught in other settings.
Since a considerable amount of our knowledge is organized by subject matter (mathematics, science, etc.), so too are our preconceptions. Before beginning instruction on any new topic, teachers need to know their students' preconceptions because learning, and therefore instruction itself, varies depending on whether preconceptions agree with the concepts being taught or contradict those concepts.